Driving 55mph? (Is it the most efficient speed?)
 

Sorry if this is in many other threads... just was reading this article on BusinessWeek, and saw these comments:

http://www.businessweek.com/bwdaily/...n_id=rss_daily

# Gau Mar 11, 2008 3:16 PM GMT Actually no, 55 isn't the most efficient speed The best ratio for gas is just above minimum rpm in your highest gear. RPMs is what eats gas, not speed... t Generally older cars run from 45-60 depending on gears and ratio. However on a lot of newer higher end cars with 5 and 6 speed gears, those number goes up...

# Tobin Mar 11, 2008 3:09 PM GMT the difference between gas savings at 70 and 55 is negligible on today's vehicles, in fact some cars in overdrive have better MPG at 70 than at 40 or 55

Is this true?

The first one about the lowest you can be in top gear is generally considered a good gauge among ecodrivers, however, speed (and therefore aero drag) is a HUGE factor and that number is no where near where he has it. I think my most efficient measured speed was 35 in 5th...in my mom's cars it's probably somewhere closer to 30.

I'd agree with SVOboy. #1 is true as per SVO's statement. #2 is false. Aero losses kill your mileage at higher speeds.

My geo definitly likes to go as slow as is practical in 5th gear. It has pretty low gearing.

There is also the effect of the engine's efficiency curve. Aero load goes up with speed but the engine may also be going towards a more efficient rpm to help offset drag (depending on gearing). I believe my GMC does this on the highway (it has a 5spd manual). It has pretty tall gearing. The scangage seems to average similar FE going either 55 or 65 on the highway. I can only attribute this to having something to do with the engine's efficiency range. Of course anything over 65 causes a decrease in mpg and speeds under 55 return better FE.

It depends on the specific model. For some cars, the difference is negligible.
http://www.greencarcongress.com/2006...onsumptio.html
http://bioage.typepad.com/photos/unc.../autobild1.png

My car has a fuel consumption curve that is flat from 30mph to 65mph, and drops only slightly at 70mph. My Cd is low enough that the slight increase in aero drag from 30 to 65mph is offset by the increase in ICE efficiency as I increase the load on it. It is actually easier for me to get good mileage at 65mph than at 30mph because at the lower ICE loadings (barely above idle), the throttle/mpg relationship becomes super sensitive. I have to use the hand throttle to consistently keep getting the better mileages at the lower speeds.

Driving 55
 

I'm also in the "it depends" camp.I think every vehicle has a sweet spot.There's no question that loads increase with speed,however,how the powerplant reacts to increased load may be counterintuitive,as in my case with the Dodge D-100.It's mileage is better above 55mph,so in it's particular case,it's brake specific fuel consumption hits the "sweet spot" at 65mph,not true for any of my other vehicles.Experimentation may be the only means to discover peak efficiency for each vehicle.

between 60 and 65MPH seems to be the most efficient for my car, however, it's also an automatic so if I slow down, it will downshift, I don't have a choice...However, I also noticed that at 65MPH I'm at 2100 but the minimal RPM for that gear is around 1500RPM but I can only go around 45MPH at that RPM....what gives? Maybe I'm just confused or something.

aerohead -

I think we need a "BSFC Topo Map Collection" sticky. It would be a cool way to compare engine behavior. There could be little "X marks the gold" spots on it for MPG, :D .

CarloSW2

My old diesel has a very flat BSFC curve from 800-2000 RPM. That plus its outrageous low-end torque allows me to think of truly radical gearing.

With an add-on overdrive I can cruise 70 MPH @ 1350 RPM and 60 MPH @ 1050 RPM. Aero mods will reduce the road load at these speeds and I'll have good roll-on capability even at ridiculously low RPM.

I would LOVE to get my hands on the BSFC graph for my engine, but it looks like OEMs don't publish those anymore. That would really help me figure out what I need to do in less than a couple of tanks, until I get an SG.

I'm pretty sure I get better mileage at 3000 rpm than 1500. I know for sure that on the highway I get better mileage at 75 than 65.

Automatic and Momentum
 

The automatic and momentum seems to hit the sweet spot around 45-50 (with gentle/rolling hills). What I have noticed is that 60 mph preserves momentum on long uphills before the ECU dumps more fuel to compensate -- depends on the grade percentage...

RH77

Thanks to getting caught in the worst blizzard around here in thirty years last Friday, I had to drive about 50 miles at my best spot - 1000 RPM in top (0.73:1 overdrive) gear. As a result of that segment I had an exemplary tank MPG. 2 MPG over normal winter mileage.

Tas:
It is good living a few years in the town where your engine was built. If you know the right guys...

Ice?

ICE = internal combustion engine

oh, ok.. I was like, why are they talking about that on here of all places..

In the performance community that stands for Intake, Cams and Exhaust :thumbup:

That would be cool.

jwxr7 -

Here's one for my Saturn (Page 10), but the resolution is a little "fuzzy" (ha ha) :

Fuzzy Pareto Frontiers in Multidisciplinary System Architecture Analysis
http://web.mit.edu/deweck/www/PDF_ar...-2004-4553.pdf

CarloSW2

neat :thumbup:. I'm trying to find one for my engine.

There's a real fuzzy one in there for the 1.0 GEO SI engine

Thanks, Wow that is fuzzy and it looks like a small peak efficiency island compared to the saturn 1.9l's.

Thanks for finding/posting that! First map I've seen for the 1.0 .

That's interesting - the best case island is higher up the RPM scale than I've been aiming for during P&G. :)

Though, having said that, the XFi cam shifts the peak island to a lower RPM...

For those with a vortech 4.3l GM engine. I found info for my truck engine here http://www.bucksengines.com/Data/24.pdf. All the way down on page 71 it has a BSFC chart. There is a wealth of other neat info there too, like % power loss vs intake air temp and intake air restriction.

sorry bbb3108 if this is taking over your thread :o.

Built for acceleration, not MPG
 

My mazdaspeed3 has 6 gears, but the gearing is target at 0-60 acceleration. I can shift into 6th at 45-50 mph, but driving at 80 mph leaves me wanting for a 7th gear. Can I avoid changing out the gears by installing larger tires?

Yes, though you'll also raise the vehicle, which may increase drag (e.g. Lexus claims a Cd .01 decrease for a 1 inch drop).

Your car and mine are vastly different, but note that by lowering my RPM 20%, my steady state fuel economy improved by about 5%. You could use that as a starting point for some rough calcs. There are many tire size calculators out there for figuring out how a circumference change affects diameter & RPM.

On all the car audio-video forums they refer ICE as In Car Entertainment.

Your engine is generally most efficient at 70-80% throttle, but you then run into the aero problem again.

Ever try to run against 45 mile per hour winds during a storm? you don't get very far, and you get tired pretty quickly. A car at 70mph is having to push through all that air, which is also pushing you back. you go slow, you have an easier time. So most vehicles (autos specifically) have a sweet-spot between 45-55 range. Generally, that spot is around 45-52ish. Also, cars usually have a hard drop in FE for every 60mph, especially in trucks and SUV's. Mostly from the wall-like windshield.

I can go a lot farther on a tank going 50, but I rarely get to do that cause I'm usually carting around a lot of impatient people. they don't mind me doing 55.

As all cars will vary, the easiest and most accurate route will be to do some experiments with FE instrumentation at different speeds.

I know automatics will drop off once the torque converter opens up, so 45mph (or whenever the TC kicks on) tends to be the best spot for those cars. My car has such low gearing that at 45mph I'd be running 1000-1200rpm, which is only 400-600rpm above idle. When I drive 55mph, my engine rpm is ~1550-1600, so I do take a pretty significant hit...but I wouldn't stay legal otherwise.

Modifying your engine for low-gearing/low-end torque might spell disaster if it is overdone. I imagine massive forces are going into a drivetrain at loads experienced at such slow speeds. This is mitigated w/ aerodynamics and LRR tires, though.

- LostCause

Huh, same here. What do you run doing 70mph? I go almost 2000rpm, not quite on the line, but close enough. I do about 1200 at 47. TC opens at 45.

Great link there. Thanks for posting.

Pete.

No problem. Too bad it's hard to find comprehensive stuff like that for other engines.

Last night I drove 3 miles across the town and according the onboard computer managed to do 61mpg. On the way I had to stop 4 times at the traffic lights and max speed was 30mph. Using engine braking wherever possible really helps to keep the consumption of a feather weight car like my smart Roadster (round 1700 pounds) very low even at urban speeds but once you raise the speed from 30mph the mpg will drop quite drastically. This morning I cruised 50 miles down the motorway at 65mph and managed to do 39mpg.

On the last few trips I've been setting the cruise at 58 or so,
just to see how it compares to the modified P&G I do in the commute to and from work.
I' going to do a table or graphic once I get a few more runs,
but the slower cruise has so far yielded about the same or better mpg than the P&G of speeding up down hill and then holding a contact throttle position going up and trying to keep the mpg's up while not holding up traffic.
This sometimes is not wise as the traffic gets upset as I speed up down hill then slow up hills.
The 58 or so cruise had much less cars 'getting around' me while going slow uphill,
and held a good mpg.
The :turtle: cruise is a lot less tiring driving too.

I think calling that a "modified P&G" may be confusing - a glide, by definition, is "power off" (whether engine of, or just idling in neutral).

Regardless of terminology, it'll be interesting to see your comparison of the 2 techniques. You should start up a new thread on it.

Isn't speeding up going downhill and slowing down going uphill DWL?

yea, I'm pretty sure, but I'm also pretty sure that the order goes in reverse, slow down going up and speed up going down :thumbup:

I listen for the sound of air rushing past the car. On a no-wind day there's a distinct increase in sound between about 60-63 mph. Of course it gets gradually louder as speed increases but there's a major increase in the low '60's. Of course every car will be different but you can listen for the sound of rushing air. More sound = more turbulence = reduced FE.

I take this as an indicator of what speed to target. Wind speed affects the sound level. With a strong headwind the sound increases happen at lower road speeds, and the opposite for tailwinds. So since I have to drive the actual current conditions I now go mostly by the sound of airflow - plus of course whatever urgency I have in getting to my destination or dealing with traffic.

I commute 60 mi. each way. Partly state roads at 40-55 mph with several miles between lights, and partly open highway posted at 55-65 mph. Also some city roads at one end of the drive with frequent lights. No FE instrumentation in my '89 car.

'You're are Correct Sir'

It is more like DWL,
I've tried the P&G but with an automatic,
going to neutral or powering the engine off isn't easy,
nor feasible or safe in commute traffic.
So my modified P&G naming isn't truly accurate and the DWL is.

I've gotten tremendous improvement by doing the DWL,
and very close to it using the slow-cruise at parts of the highway where it seems to make sense.

It's really helpful when you have a leaky door and you hear wind noise :D

Mine is my leaky door :p

my trucks engine seems to relax a bit easier with the vacuum guage right around 60mph and at 55 it strugles just a little bit.. (less vacuum)

I have a 2.4 dohc 16v engine thats built for torque.. (long intake runners)

Hey, I've got one of those instruments too!