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Old 04-23-2022, 03:08 PM   #11 (permalink)
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The prius "transmission" is essentially a cvt in that the motor generators hold the engine rpm steady at a given throttle position/load while speed varies. So you won't get lower rpms from larger tires.

People do put larger tires on the prius to increase ground clearance, but they always lose mpg doing so. Some of that is because their odometer reads low, but larger tires are heavier & increase the frontal area. People wanting more ground clearance also tend to pick more aggressive tread patterns that don't have as good of a rolling resistance as eco-focused choices in the factory size.


PS: going back to your first post you talked about the distance you can drive in electric mode being a proxy for how the traction battery is doing. Since you sometimes drive on dirt, I thought I'd point out that my gen 3's newish battery can go about 2 miles on pavement at ~15mph, but struggles to go 1/2 mile on gravel/dirt at ~10mph. The rolling resistance is that much worse. Larger tires might actually help slightly on gravel for the same reason a 29er tends to roll better off road than a 26" mountain bike.

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Old 04-23-2022, 04:26 PM   #12 (permalink)
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The CVT in the prius still has a max high gear ratio. I'm not sure how the computer calculates what gear ratio it runs at, but it assumes stock sized tires. My thinking would be it would use higher gearing while in cruise when it should be in the highest effective gearing already. I've read the gear ratio of it is 4.11, I suspect that's the sun gear ratio. The "CVT" effect is just engine vs electric motor rpm to simulate the lower gears. A side effect of this is the 41mph max speed to kill the engine and have the electric motors be happy (regen or driving on electric only) can be increased by the % of the oversized tires as the rpm's for the engine/motors wouldn't change besides the effective final drive gear ratio. I'd hope the tire size change doesn't make the computer use higher than ideal ratios for taking off and such, but who knows if the prius's programming is based purely on best mpg figures for a given acceleration rate. I liked my corolla because I could give it a lot of throttle but the transmission wouldn't down shift, so I could load the engine well and put it in the more efficient range. It wasn't a fast take off since I'd let off and get it to shift basically as soon as possible. The prius seems to rev up the engine more than ideal, it doesn't sound like it's loaded as much as I'd like, but I don't have direct control over that either besides what I tell it via the gas pedal.

My battery testing was done at 30mph on pavement, my house is 1/4 mile on dirt and I travel 10-20mph depending on road condition. Thanks for the extra details about the actual speed you traveled at, I think roughly speaking that should be a reasonable way to test the hybrid battery, 15mph should have very little areo effect and the loads should be more or less linear as long as the road start and end points are roughly the same elevation (perfectly flat would be ideal of course). My dirt road can be pretty smooth and just a few loose stones on top but otherwise about as hard as pavement, other times it's loose like right after it's graded, and of course when it's wet it can be muddy. The road as a lot of sand/dirt not much rock. I suspect the scan gauge or the app suggested before would give a solid value as well, but sometimes real world testing is nice since the other numbers should be more theory based (calculated, but effectively a guess based on past performance).

Based on a little searching, your 2010 should have a 1.6kwh battery, while mine seems to be around 1.3kwh which is about 23% difference. That should mean if I have the same usable % of the battery as you, I should be able to go roughly 1.5 miles at 15mph. I know the 2010 is a bit different from the 2004-2009 era. I suspect mph on pure electric mode effects the efficiency point of the electric motors (rpm vs torque output). I haven't studied electric motors a whole lot, but peak levels if I remember right are around 90% efficient (1000w of electricity in for 900w of mechanical work output). I'm not sure how much that drops at less than ideal rpm's though and these cars have a bit of a unique electric motor in them.

Anyway, the gravel road is only 1/4 mile (1/2 mile per round trip), so the overall effect it has on mpg shouldn't be much. All the other roads are paved even if they are bumpy. I can really feel the bumps, but I'm guessing that's more because of the broken strut spring lol.
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Old 04-23-2022, 09:39 PM   #13 (permalink)
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Quote:
Originally Posted by ps2fixer View Post
The CVT in the prius still has a max high gear ratio. I'm not sure how the computer calculates what gear ratio it runs at, but it assumes stock sized tires. My thinking would be it would use higher gearing while in cruise when it should be in the highest effective gearing already. I've read the gear ratio of it is 4.11, I suspect that's the sun gear ratio. The "CVT" effect is just engine vs electric motor rpm to simulate the lower gears. A side effect of this is the 41mph max speed to kill the engine and have the electric motors be happy (regen or driving on electric only) can be increased by the % of the oversized tires as the rpm's for the engine/motors wouldn't change besides the effective final drive gear ratio.
Ah I see - the larger tire size should trick the car into thinking you're only going 40mph when you're really going 42 (or whatever).

As far as I can tell, the effective gearing is pretty similar between gen2 and gen3 despite large differences in final drive & MG2 rpm:

NHW20:0.620m(185/65R15)
ZVW30:0.635m(195/65R15)

NHW20: 6400rpm / 4.113 = 1556rpm (max wheel rpm)
1556rpm*0.620m*3.1416*60min/1000 = 181.9 km/h (113.0mph)

ZVW30: 13500rpm / 2.636 / 3.267 = 1568rpm (max wheel rpm)
1568rpm*0.635m*3.1416*60min/1000 = 187.6 km/h (116.6mph)

Quote:
I'd hope the tire size change doesn't make the computer use higher than ideal ratios for taking off and such, but who knows if the prius's programming is based purely on best mpg figures for a given acceleration rate.
I think the computer just tries to set the ICE at the most efficient rpm for the given load and lets MG1/MG2 figure out how to make that happen for whatever speed you're currently at.

For the Gen2, you can stay at the 1000 rpm "idle" until about 66mph and hit 110mph at ~3,000rpm so I don't otherwise see a benefit from the tire gear change. You can play around with MG1, ICE, and MG2 speeds here:
Toyota Prius - Power Split Device


Quote:
I liked my corolla because I could give it a lot of throttle but the transmission wouldn't down shift, so I could load the engine well and put it in the more efficient range. It wasn't a fast take off since I'd let off and get it to shift basically as soon as possible. The prius seems to rev up the engine more than ideal, it doesn't sound like it's loaded as much as I'd like, but I don't have direct control over that either besides what I tell it via the gas pedal.
Look again at the BSFC graph:


The prius ICE is meant to operate on that thick black line - varying rpm to match the desired power.

Quote:
My battery testing was done at 30mph on pavement, my house is 1/4 mile on dirt and I travel 10-20mph depending on road condition. Thanks for the extra details about the actual speed you traveled at, I think roughly speaking that should be a reasonable way to test the hybrid battery, 15mph should have very little areo effect and the loads should be more or less linear as long as the road start and end points are roughly the same elevation (perfectly flat would be ideal of course). My dirt road can be pretty smooth and just a few loose stones on top but otherwise about as hard as pavement, other times it's loose like right after it's graded, and of course when it's wet it can be muddy. The road as a lot of sand/dirt not much rock. I suspect the scan gauge or the app suggested before would give a solid value as well, but sometimes real world testing is nice since the other numbers should be more theory based (calculated, but effectively a guess based on past performance).
Yeah, road surface can have a huge effect on how well it rolls. Hardpack dry dirt is probably even better than asphalt. Smooth concrete is probably the best you'll encounter. Potholes and water and gravel and mud all add resistance - sometimes 2 to 3x the rolling resistance which is all the really matters at battery speeds...

Quote:
Based on a little searching, your 2010 should have a 1.6kwh battery, while mine seems to be around 1.3kwh which is about 23% difference. That should mean if I have the same usable % of the battery as you, I should be able to go roughly 1.5 miles at 15mph. I know the 2010 is a bit different from the 2004-2009 era. I suspect mph on pure electric mode effects the efficiency point of the electric motors (rpm vs torque output). I haven't studied electric motors a whole lot, but peak levels if I remember right are around 90% efficient (1000w of electricity in for 900w of mechanical work output). I'm not sure how much that drops at less than ideal rpm's though and these cars have a bit of a unique electric motor in them.
I'm new to motors and batteries so I'm learning as we go. My understanding is the gen2 and gen3 both have 28 7.2v cells totaling 201.6 V with 6.5 amp-hours capacity. 201.6V x 6.5Ah = 1310 watt-hours, or 1.31 kWh.

From my reading gen3 cells were slightly improved and had less internal resistance for better efficiency.

Quote:
Anyway, the gravel road is only 1/4 mile (1/2 mile per round trip), so the overall effect it has on mpg shouldn't be much. All the other roads are paved even if they are bumpy. I can really feel the bumps, but I'm guessing that's more because of the broken strut spring lol.
Yeah, I just hate when my ICE is on while I'm poking along on a dirt road because my mpg plummets. I prefer the engine warm up and/or charge the batteries while I'm driving 30-40mph...
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Old 04-24-2022, 01:53 AM   #14 (permalink)
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That site is interesting, one thing it doesn't talk about is the electric draw of the electric motors though. If I remember right MG1 is the larger drive motor while MG2 is the starter/alternator/smaller one. Like if I set the ICE to 1100 rpm, it requires MG2 to run at 1500rpm to keep MG1 at 0 rpm which ends up being 26mph.

If I re adjust the figures to get something logical for 55mph while keeping the ICE at 1100 rpm, it has to turn MG1 at -4500 rpm and MG2 has to spin at 3250. I'm pretty sure at those figures it would have to be heavily using the battery since I'm pretty sure the ICE doesn't have enough hp at 1100 rpm to make the car go 55mph. I don't have a tach/scan gauge to validate, but it sounds more like 1500-1800 rpm in cruise. Bringing up the ICE rpm allows the MG1 and MG2 rpm's to be lower.

For the actual control, I'm not sure if the computer is doing the math on the fly for the given load, but I suspect it's more likely using lookup tables like the fuel map tables and the computer is tuned via that table to give desirable results. My guess would be the table route which goes back to the idea of if you increase the tire size, it will be like going up hill for what the computer sees, or going against a higher wind speed. Pretty sure that would mean the throttle is pressed more, but you'd be going at a slow mph according to the computer than what the real speed is.

Really not 100% sure what the overall effect would be. I think using a scan gauge to monitor rpm and such, swapping tires out and seeing the effect is about the only way to really know for sure. The whole system is a bit counter intuitive compared to a normal car.

For the BSFC graph, that's exactly what I figured the bold line was for, but thanks for clearing that up. Seems like the target for take offs would be to target the lower end of the gray zone to be in the most efficient range. I suspect there's a point in the gray blob that's more ideal than the rest, but maybe the effect isn't isn't to really measure.

I've been looking around on google maps for a good area for a test section of road. There's two main options that I know has very little traffic, one is 2 miles, the other is 2.5 miles. I'll have to keep poking around, I'd like to find around a 5 mile stretch of road and do steady state mph vs mpg for my car in mostly stock form since I think that will give the best base line vs basing it on take to tank fill ups. If I understand things right, the displayed mpg figures don't get calibrated at all, so it should be a consistent reading I would think. I'm used to my corolla where the best mpg I could possibly get from the car was around 45mph (40mph was best but hard to keep it from down shifting on small hills and such). The prius on the other hand I'm not super sure what the best travel speed is. I've seen people say there's a sweet spot around 57mph and another said something like 62mph. From the tiny bit I've played around, it looks like 45mph gets much better mpg than 55mph and is likely how people can hit 50mpg+ with this generation of prius. It's really crazy how consistent the numbers are on this car though, like you can just about predict the average travel speed purely from the mpg figure from what I've been seeing so far. Of course modded cars throws that logic out the window a bit.

If putting larger tires on the prius makes everything else the same besides the extra spinning mass, then I suspect the only room for mpg increase would be smaller tires or narrower tires and it's main effect would be stop and go traffic which isn't what I'm doing. I'd dare to say if the same 1.5L engine was paired up to a normal manual transmission, I could get as good of mpg with a little work keeping the engine rpm in the right ranges, at least for constant speed cruising. The CVT setup is a neat concept though, seems like it's a very reliable way to design a transmission too.

For gen2 vs gen3 hybrid batteries, I just searched it a bit, sounds like they are the same internally for the main battery modules, same voltage and number of cells so same capacity. Google's first result that gives the instant answer in results gave the wrong figures. It's a per-release post before the 2010 came out, so I guess maybe there was a rumor of a 1.6kwh battery that was incorrect or something.

NiMh is 1.2v per cell, what you're calling a cell is actually a "module" as one site put it. There's 6 cells per module, 28 modules (from your post but pretty sure that's right), so 168 total cells or a nominal voltage of 201.6v. I don't know NiMh batteries too well, but I've done quite a lot of reading up on Lithium Ion and similar tech batteries. 3.6v nominal voltage, 4.2v is fully charged, and around 3v is fully discharged. Here's a Lithium Ion discharge chart. 1C is 1x capacity of draw, so if it's 2200mah rated cell, then 1C is 2.2 amp draw. Lithium Ion does really well at low and high discharge rates, but lower should give more capacity as less heat is generated. I've hread NiMh does very well with high amp draws and Toyota designed these batteries to sit between something like 40% and 80% state of charge so the battery will last an extremely long time. I suspect the NiMh batteries last longer than the Lithium Ion versions even though that's the latest and "greatest" tech. I've read that charging NiMh is a little more tricky, I don't remember why though. I was looking for "cheap" batteries to use for backup power sort of like a power wall. Eventually I'd like to get solar panels and go effectively off the grid. If I read my power bill right, the only fee I'd have to pay to stay connected is $7/mo just to have the access to the power lines. There's always going to be maintenance and such, so it probably wouldn't be a bad idea to stay hooked up just for the "backup" power if/when my main power doesn't come from the grid. The prices have jumped up a lot lately before the fuel price increase so it might be quite economical for DIY solar installs. Good figure in my mind is 10% ROI, or 10 years to break even. Lead acid batteries taken care of well generally lasts for 10 years and batteries are a major % of the initial cost. Kind of getting side tracked, but someone might find it interesting at least.



Also I ran over your math, you got it all right, 1.3104kwh to be exact. Seems like most people round to 1.3kwh though.

I find it interesting that the electric motors in the 2nd gen prius can make around a max 60kw of power (80hp), but the battery pack max output I think I read was 36kw or 48hp. Still quite impressive, about 27.5C discharge rate. The highest output Lithium Ion cells that I'm aware of (for tool batteries) are max output of 40C.

Since the dirt road is right at the start/end of my trips I don't see that effect much. Of course the first couple miles the mpg is pretty horrible since it's recharging the battery. It would be interesting to dig up the right numbers to calculate how good or bad driving purely in electric mode is. A while back I took some real world figures of people testing the prius as a generator (1kw load on the 12v system) using an inverter and accounting for the inverter losses and fuel used, it was very comparable to to the large backup diesel generators that are for power backup during peak grid usage which are generally loaded at the ideal amount for the best power output per unit of fuel burnt (80% load of capacity or so if I remember right). It's insanely more efficient than a 5hp 5kw generator, I think it was 2-3 times more efficient than a Honda 1500w generator at max efficient load.

Anyway, I suspect we need charge vs discharge efficiency of the NiMh cells, inverter efficency, ICE (at the ideal load), MG1/MG2 efficiency for generating and discharging. There's a lot of steps there, so I suspect the 66% figure from before is probably a good estimation.

Some googling around I found something that looks good. They basically say 100% efficiency for charging, 80% for discharging, average that to 90% for easier math.

NiMH Charge and Energy Efficiency ยท AA Cycler

This is a real interesting chart that came up while searching for other numbers. Looks like under ideal conditions, the motor/inverter combo can hit 95% efficiency, the low end is 65%, but it looks like most of the area is around 90%. 90% is a pretty solid number when calculating efficiencies of electronics if they are designed well, I was expecting 90% for the motors, and 90% for the inverter (90% of 90% is 81% overall).

https://www.researchgate.net/figure/...fig3_346897527

Using those numbers and assuming a 90% for electric motor + inverter, that leaves us to the ICE efficiency (thermo efficiency). First google result says mine is about 37%, same site says your's should be 38%.

https://www.greencarcongress.com/201...-20110411.html

1 gallon of 87 octane gas holds roughly 33.7kwh worth of energy. 37% drops it down to 12.496kwh (12.806kwh for 38%) for the output of the ICE as mechanical work to the electric motor(s) for charging. 90% is 11.222kwh into the battery and 80% drop from the discharge of the battery 8.98kwh back to the electric motor to move the car 90% again 8.08kwh. Overall efficiency 23.98% Ignoring the engine's number since either case the engine is making the power to move the car or charge the battery... we get round trip efficiency of power going into the battery and back out at 64.8%. That's really close to the 66% figure before (whoever did the math probably used the 95% figure instead of 90% for the inverter/motors).

Now for the real interesting question... the prius engine is roughly 37% efficient but clearly that's under ideal conditions, just how much does it drop when it's in the warm up phase. I'd assume while the engine is warming up, it should be using the battery as much as possible until the engine is warm enough to have a high enough efficiency to be worth while recharging the battery back up.

https://www.anl.gov/article/plugin-h...ing-efficiency

Looks like a room temp engine (22C or 72F) uses roughly 25% more fuel than fully warmed up. Pretty interesting study, looks like for the exact engine, it takes quite a long time for the oil to come up to temp. On the Toyota Camry, there's a heat exchanger on the oil filter to the engine coolant, I guess that design is to help with quicker warmups of the oil to give better mpg.

So 25% more fuel used (or 25% lower thermal efficiency for the engine, so 27.75%) vs let's call it 65% efficiency of a fully warmed up engine charging and discharging the battery for the mechanical work which works out to be about 24% efficiency overall.

Lot of numbers thrown around and such, but if I didn't mess up anywhere, a cold engine is still more efficient than charging and discharging the battery for electric only mode by almost 4% (purely theory based). Once the engine is fully warmed up, it's 13% less efficient. In other words, if this math is all right, driving the last 1/4 mile on engine power would be more efficient than on electric by quite a bit, so pulse and glide would probably make massive improvements for that short leg of the trip, another way to trick the engine into running would be to turn the HVAC system on and make it demand the engine to be on. AC would be less than ideal but I suspect that's the easiest way to get the engine to stay on for a bit.

There's some small numbers I'm missing in all of this, the steady state average mpg for say 10mph, 20mph, 30mph on a trip long enough to run on the battery out of charge or use some trick to force the car to drive under engine power. If the mpg is bad enough, maybe at those low speeds it's more efficient to go the battery charge route and the ideal thing to do is to actually drive faster. Nice and counter intuitive again lol.

It's been a while, but based on my experience with my Corolla, it did quite well at low loads. 45mph was roughly the best mpg, 35mph it gave very good numbers still compared to the 45mph number. I didn't drive it under 35 very often so I don't recall how much the mpg dropped at lower speeds. I know when it was cold started from my parent's house, I always targeted hitting a reasonable mpg figured by the first corner (1/2 mile down the road, slightly down hill). It's been a long time since I did that, I think it was around 45mph I'd get the car up to then coast in neutral till the stop sign and I could hit 35mpg+ on the scan gauge. 25% of 45mpg is 33.75mpg, so I guess I was targeting about the same efficiency as my normal driving but with a cold engine (25% less FE). Really crazy how all of these numbers are working out so well.

Anyway, all of this math and thought experiments gives me some ideas. Of course low hanging fruit first, fix the front end of the car up, get the rear strut fixed (might lower mpg since it will sit higher), grill block(s). Belly pan should be a good item too, basically try to insulate the engine so it warms up faster, but not restrict air flow too much so the radiator can still do it's job, hood seal helped a ton on my corolla to keep heat in the engine for longer. Seems like the most gains on a prius are more engine/trans/heat related than the really easy areo stuff.
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Old 04-24-2022, 04:05 AM   #15 (permalink)
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Some old charts people posted on Prius Chat back in the day.

This one is for a 1st gen prius and I believe shows how close the prius was operating to the theoretical optimal rpm/load:



This was calculated vs actual mpg for the gen1 prius:

The "marathon" data points were using pulse & glide. I think they calculated the gen1 prius had ~500watts of fixed load powering all the computers and such (not including A/C) which is why it isn't very efficient to crawl around at 5mph.


Gen2 steady state speed vs mpg:


PDF describing the Gen2's warm-up stages:
https://web.archive.org/web/20100206...ence051408.pdf
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Old 04-24-2022, 02:50 PM   #16 (permalink)
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Thanks for the graphs. That last one I swear I posted, but maybe I didn't add it to my reply.

Interesting discovery on my trip to town and back today. 55mph I got the same typical mpg, once fully warmed up I was seeing around 50-53ish mpg roughly for an average (I didn't reset the trip). I bumped it up to 60mph and wow did the mpg drop, looked to be around 43-45mpg. On the highway I went 70mph to test higher speed and I got about the same as 60mph. I basically ignored the mpg once I got in town and just drove, coasted where I could, etc (50mph to go with traffic, I was pressed for time a little). I did notice the mpg seemed a lot higher at 50mph compared to 55mph though.

On the way home, I took the backway home to try out steady state speed of around 45mph just to see what the effect is. All I can say is wow.... that last graph lines right up with what I was seeing, right around 70mpg I'd guess for the average, maybe slightly less. The graph shows I should see about 75mpg at that speed.

Clearly I have some repairs to do to the car yet, but based on that, I'd say the gas engine should be pretty healthy yet, at least for the lower loads. The slight miss I hear at idle might be effecting mpg at higher loads. Another thought is the areo of the body is messed up a little in the front, so that could be an effect as well. I wouldn't think it would be a 9.5% drop at 45mph but around a 23% drop at 55mph. My testing at 50mph wasn't super great, but I think I was seeing around 55-60mpg and the chart suggests it should be around 62mpg. Taking the worst case, that's about 9% difference again.

I just got my parts in today to fix up the car, so getting the rear end back to normal height (it's down something like 3 inches) might help or might hurt mpg. I swapped out the cabin filter (nasty, plugged up pretty bad, etc), and the air filter (wasn't too bad) already.

I wish there was well documented speed vs mpg charts for all cars, it makes knowing how to drive them for peak mpg a lot easier.

If I recall correctly, the trip mpg before I started was 45mpg and when I got back home it was almost 48mpg, so clearly that trip was much more efficient vs past driving. I'm so tempted to reset the trip, but I'm wanting to hand calculate the mpg to validate how accurate it is. I know the gas tank on these cars are a bit tricky to get consistent fill ups, so I guess it's not a huge deal. Need to get my scan gauge setup so I can see trip vs tank mpg.

Anyway, I have 100 miles so far on this tank, still quite a long ways to go before the next fill up.

Hopefully I can get my numbers within about 5% of the chart, the car does have a bit of miles on it, so it would make sense it's not as efficient as it was when new, but I'd say there's clearly an issue related with 55mph+ vs 50mph and less on my car.
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Old 04-24-2022, 05:56 PM   #17 (permalink)
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The ScanGauge 2 does have some X-Gauges for the Prius 2. Here are a couple links to get you started.

https://www.scangauge.com/x-gauge-co...ius-2004-2009/
https://priuschat.com/threads/gen-ii...t-temp.117335/

Side note: After the SG2 is "trained" for your car, the mpg readout will be more accurate than the one in the Prius. The mpg readout in the dash is a bit optimistic.
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Old 04-24-2022, 10:32 PM   #18 (permalink)
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I have driven both the 2nd gen and 3rd gen Prius quite a bit - 50-60 mpg in the 2nd gen and 60-70 mpg in the 3rd gen average. For the 2nd gen, accelerating with the instant mpg at half your current speed in mph seems to give the greatest accelerating efficiency. For the 3rd gen, accelerating with the power level just before it enters the red zone offers the greatest acceleration efficiency. Read both of those tips either on here or on PriusChat, and they seem to hold up from my driving.

The 3rd gen Prius has a nifty feature where the physical brakes aren't pressed much (or at all?) if the regen bar isn't maxed out on the display, so efficient braking does its best to never max out the regen bar. Unfortunately the 2nd gen Prius doesn't have a display feature like that so I typically just coast in gear with my food off the pedal with that little bit of regen dragging.

The biggest downfall of the Prius on short trips is the stupid emissions feature that makes the engine idle high for the first 2-5 minutes of operating yet only uses EV to move the car unless you floor it. To solve that I typically use P&G and put the car in neutral at lights and while coasting until the car warms up to save on gas. So short, 2 mile trips in the Prius are now 35 mpg rather than 25.

For both cars 45 mph cruise control has always yielded the best fuel economy. Best trips around 68 mpg in the 2nd gen and ~75 in the 3rd gen. Never was able to get P&G to work but that supposedly does better. 55-60 on highway is best, 70 and above definitely reduces fuel economy by a lot. Pretty standard for most cars. Speeds under 30 mph EV only always if possible, 30-40 mph was hit or miss depending on the road and battery charge level, it would either be 30-40 mpg or 80-90 mpg. Seems like in the 30s mph is where it generally tries to use a higher load to charge the battery to get better engine efficiency per the BSFC charts. Possible to get better mpg there than at 45 mph, but I've always found 45 mph to be the most consistent.
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Old 04-24-2022, 11:01 PM   #19 (permalink)
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Yea I figured the scan guage would be more accurate since it's calibrated at fill ups. Still haven't ran across mine yet. I did borrow a scanner and I had a P0420 code which is cat inefficiency code. Possible secondary o2 sensor failing or converter going bad/plugging up, etc. I've cleared it for now, will see if it comes back or not.

Oil change done on the car, man that oil filter was super tight for whoever put it on last. Filled with full synthetic 5W30. New plugs, old ones looked good and I double checked with my dad (he's a mechanic) and he said they look fine. New air filter and cabin filter so the basics are covered. Rear struts were interesting, basically had to take most of the plastic off the rear of the car to do the job. Found out BOTH springs were broke. Now with new struts it sits something like 6in higher, I figured they sat lower for the econ. Anyway, I did some limited testing at 55mph and the mpg seems to work out about the same. Best round the block mpg my dad and I got was 53mpg but that one had a bit of 45mph speeds in it, wet roads which I wouldn't think would effect things too much.

I took the car to a test area, fairly flat ground 2 mile stretch of empty road. Stopped at the corner, reset the mpg trip and tried taking off a few different ways including the return trip (4 miles total). The one pulse and glide vid said to take off with the mpg and mph matching each other, In my little test I found that was the worst, round trip 57mpg @ 45mph. Taking off lightly targeting 1500-1800rpm seemed to give the best result, 60mpg @ 45mph, and taking off fairly hard, guessing around 2500rpm, maybe a bit higher and it ended up at 59mph @ 45mph. Seems like the take off doesn't matter too much. I coasted and turned around as close to the same as I could for all of the runs. The main focus was 45mph mpg in more or less ideal conditions (using cruise to be more consistent). On the way back I reset the trip while going 45mph and saw 70mpg average before the corner (by a lake, so likely a bit down hill) and by the time I got to my corner (about 4 miles) it dropped to 60mpg.

Oh, I also discovered more issues with the car. Driver's side wheel bearing is quite bad. Passenger inner tie rod is a bit sloppy, hard to really tell on the driver's side with the wheel bearing being so bad. I suspect that's part of why the mpg isn't quite as good as expected. I'll have to get the wheel bearing ordered and install it once it comes in. I don't think this is dropping the mpg based on the speed vs mpg chart. The way it's looking, it might just be engine wear causing it, I'd guess mainly rings.



Those mpg vs mph numbers for take off is pretty close to what I worked out. I basically target an rpm range on the engine (guessing, no gauge yet) and it's roughly 1/2 the mpg as the mph. I agree that it seems like the cold start setup is pretty stupid on them, even just wanting to move the car a short distance on EV only mode you can't really do that unless it takes under 30 secs. I'm not sure, but it seems like driving it under 30mph when cold it's running purely on EV mode and the engine isn't doing much. I guess the concept is to warm up the engine with no load for emissions, even though that's against mpg figures as I understand it.

I'm glad I'm not the only one struggling to hit much over 25mpg for the first 5 min graph. I might see 30mpg. Second graph with my driving current driving style seems to be real close to 50mpg, but I'm "cheating" a bit by driving 45mph. My goal is 50mpg at 55mph which I think is achievable assuming the engine isn't worn enough to hinder things. I'll have to pull a spark plug some time and do a compression test on it. That should be an interesting thing to try to do lol.

So far it seems like nothing I've done has changed anything besides slowing down increasing mpg which is a pretty well known thing. I suspect the grill blocks and fixing up the belly pan should give at least a slight increase. It looks like the rear bumper can catch air a bit as well.

I tried the 15mph battery test on EV mode only, it didn't quite hit a mile before the engine kicked on, but the battery charge indicator didn't change at all which is weird. That was right after the warmup cycle on the engine, so might have been a bit early to test. I suspect a weak battery or bad modules could cause poorer mpg as well. Either case, even getting 45mpg, that's a lot better than my other vehicles. LS400 needs work, but it's around 20mpg in the summer. The Matrix I couldn't get very good mpg with it, best was 33mpg for a tank driving somewhat normal with coasting and such.
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Old 04-24-2022, 11:36 PM   #20 (permalink)
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What makes mpg testing difficult on the Prius is that the Prius targets a specific SOC (state of charge) for the high voltage battery. On the 2nd gen Prius, this is 62.5% (according to Dr. Prius), represented by 6/8 bars lit up on the gauges. If the battery SOC is > 63% the car will use more battery to get it back down to 62.5%, and if the battery SOC is < 62% the car will use more gas to charge the battery back up. So unless the starting and ending SOC of the battery is the same, or within 1-2%, any MPG findings are going to be skewed unless run over several tanks.....which is nearly impossible in a 2nd gen Prius due to the catastrophe that is the bladder gas tank....who ever thought that was a good idea to put in a car ever?

Tire pressure was probably the single best thing we did for the Prius, inflating the tires from 30 to 55 PSI made a world of difference. No noticeable difference in ride quality either, unlike my current Insight that is harsh as hell at 60 PSI.

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