Prius 15's are better than aftermarket 17's! (Wrong wheels/tires can wreck your MPG)

[cwstnsko]

Most of the spec charts list the rotations per mile for each size of tire. To the extent that these are accurate, they are the single easiest way to compare tires of different sizes. In the case of the original poster, the Goodyears are rates at 848 rot./mile, the Bridgestones are rated at 853 rot./mile. Which brings us to the same conclusion through yet another method that the difference is negligible :-)

I have also used the rotations per mile to help me choose tires that will correct speedometer/odometer errors. In my experience, a few less rotations per mile than stock tends to make the gauges more accurate, but GPS makes it pretty easy to do the math.

[wyatt]

I just added an entry to my fuel log (but it's "old", from a little over a week ago). It was a tank with about half interstate and winding mountain roads, and the other half to and from work. I got a respectable 52.5mpg on the tank. Based on the readout from the MID, and it's relative accuracy, I am guessing the 64mpg tank cut out early, and my guess is it was actually closer to 55 or 56mpg, similar to what the MID was reading. So far this tank (to and from work only) I am averaging 56.0mpg according the the MID, looking forward to seeing how the tank turns out, but not so much looking forward to filling up .

[wyatt]

Originally Posted by LocoJason

I don't see anyone having asked about the offset of the wheels as well. If they were a wider wheel you obviously have a larger frontal area, but if the offset of the wheel is lower (as aftermarket wheels tend to be) the wheel will sit further out... it will alter the wake and cause a large disturbance in the airflow along the sides of the car.

I would almost be willing to ask a Prius owner to throw on some 1.5" wheel adapters with even the stock wheels and tires. I have no doubt this subtle difference would have a much more drastic effect on a vehicle like the Prius, which is highly optimized for aerodynamics (among other things).

Sorry, I seemed to have missed answering this question. The offset, though I don't know the numbers, doesn't seem to be any different than the 2010 Prius rims I have on the car now. The rims were tucked in nicely, not sticking out in the wind like so many after-market rims that I see. With that said, I would still bet that the aerodynamics of the rims was pretty bad compared to the current rims with hub caps. How much worse? I don't know, but maybe 3 or 5%, just based off seeing other's improvements and making a semi-informed guess.

[MetroMPG]

Any chance you have a pic of the aftermarket wheels?

[wyatt]

Originally Posted by MetroMPG

Any chance you have a pic of the aftermarket wheels?

Sorry for the delay. I have remembered to do this at the least opportune times .

I got a picture today of the Prius with new 2010 rims parked in one of the two LEV (Low Emission Vehicle) parking spots in this parking lot. Yes, that's right, if you drive a hybrid, and you are at work early enough, you can park right up next to the door. In the summer I tend to park in the shade and walk, but in the fall/winter/spring, a toasty car is welcome .

This is what it looked like before the rim-swap. I know the rolling resistance is better and the aero is better, but it doesn't stop me from wanting to get some aluminum pizza pans... I am thinking they would be super easy to attach to the hub caps. I was also thinking that if they are attached to hub caps and I put ventilation holes in them, it should be easy to swap the hub caps around when rotating tires. "Directional hub caps?" you say? YES! I am thinking something along the lines of the rims on the FT-Bh, only with fewer openings (probably 5 to match the 5 openings in the rims as they are today...)

[ECONORAM]

Originally Posted by wyatt

I will have to check what the tires were when I get back home, and report back. I do know the rims were 8" wide, vs the 7" wide Prius rims. The rim/tire combos are noticeably lighter when lifting. This is comparing between my wife's 15's and the 17's. I don't have a mass reading on both, so it's not a real comparison, but it is something I noticed.
Running the old tires on the back of the car would be an option, but we plan to sell off the 17" rims to someone looking to "upgrade" their ride. Anyone here looking for some 17's?
I agree, it could be a large number of factors that impacted the large change in FE. I am just glad it is up, way up .

I'd weigh each of the wheel/tire combos. I think you will find the 17s are heavier by a few pounds. Each. I mention this because I've been looking for some different wheels for my truck. Factory 20s are 42# each, I'm told. Just the wheel itself. I plan to pull one off sometime and put it on the scale. (by the way, my factory tires are 42# each also!)
My tires have 51K on them, but weighing should give me some sort of idea. I can get some 20" Centerline forged wheels that weigh 26# each. Their 20x8s weigh 26#, 20x8.5s weigh 28#! A lot of guys on DodgeTalk that race their trucks switch to 17" wheels to save weight (and reduce their gearing). Factory 17s for my truck are about 27# each...without tires installed.
Rotational weight is an ecomodder's enemy.

On Ecopia tires, we just put a set on the wife's Avenger. Supposed to be really good. Firestone says we'll save over $300 in fuel over the life of the tires. I've pumped our tires up too; they only inflated them to 29 psi? Car says 32. I'm thinking I'll go with 35. Didn't want to replace the old tires (only 46K) but I hit a piece of metal that cut half-way across the tread on the left rear. Fortunately it was a slow leak; not a rapid puncture. The car's steering is noticeably lighter with the Ecopias.

[F8L]

The fuel economy loss associated with upsizing wheels on the Prius is very common and occurs in all cases that I am aware of. I've tried dozens of different tires and more than a few wheels in an effort to better understand this loss and try to reduce it. Unfortunately you will always suffer the loss.

In an effort to back up the OP, the OE tire size for the 2004-2009 Prius is 185/65/15 and the OE tire is Goodyear Integrity with an 855 revs/mile. The average 215/45/17 grand touring tire has a revs/mile of 844. The tire is taller but even when correcting for tire size and its effect on the odometer, the loss is still vey evident. It becomes even worse when comparing the 17" tire to an ultra-LRR tire like the Energy Saver A/S.

I performed quite a bit of testing on my 2012 and I found the FE loss to be able 3mpg when going from the OE 15" tire (Yokohama S33D 195/65/15) with 829 revs/mile to a 215/45/17 Primacy MXM4 LRR tire with 844 revs/mile. When I tried the Michelin Energy Saver A/S 195/65/15 (832 revs/mile) my average mpg went up by by about 3+ vs. the OE 15" tire. When I switched back to the 17" tire I lost 6-8mpg. I backed this up with several more tire/wheel swaps.

Interestingly, my first set of 17" wheels weighed 15lbs which are 1lb lighter than my OE 15" wheels. When I switched to a set of 17" Lexus OE wheels the mpg hit stayed about the same despite the wheels weighing 24.25lbs! Obviously 17" tires weigh more than 15" tires (18lb avg. vs. 22lb avg.) but the weight difference doesn't seem to have a huge impact on mpg. I personally feel it has more to do with tire design. The 17" tires suffer more from hysteresis than the 15" tires.

Anyway, I just thought I'd share my opinion because I don't know anyone who has tried more wheel/tire swaps on the Prius than I have and I still can't solve this issue without throwing ridiculous amounts of money at the problem. So for now, if you want max FE, stick with the OE 15" wheel and a set of Energy Saver A/S tires! The Ecopia EP422 is pretty good too. Maybe well get lucky like Europeans and get the new Ecopia EP001s.

[F8L]

Oops, I forgot to mention I also tried 2 sets of 205/50/17s on my 2012 as these are the same size as my OE 195/65/15s. The mpg loss is still there.

[RedDevil]

The sidewall on the 17" tires is an inch less high than on the 15"s.
On impact with the road they have to flex deeper than the 15", and it also takes more force to flex them in the first place.

The 17" rim has a much wider open gap, and the thin spokes do little to prevent the air from flowing in. The 15" rims have thick spokes and little gaps between them, so less drag.
4 big bucket sized holes generate more drag than 20 tea cup sized ones.

My guess is that setting the pressure to max sidewall PSI will have a big effect on rolling resistance, as the sidewalls will have to flex much less then. Steering will be lighter and more precise. Grip and comfort stay behind in the parking lot.

Covering the rim with a plate would help reduce drag at higher speeds.

[RandomFact314]

This inspired me to look up the Ecopia tires, are they seriously $235 a tire? The LRR Goodyear Viva II tires I have been using the past (since I've been here) years are only about $70 a tire and I've never had any issues with them.

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Update: I actually re-put in my tire size and it went down to $114 a tire. After 4 tires I dont think they will pay themselves off in extra FE over the VivaIIs