Better mileage at 70 vs 65 ?

[Vikas]

This sounds somewhat counter-intuitive but based upon my observation, my car seems to be giving better gas mileage at slightly over 70 than under 70. I have not compare it against the 55 mph but between say 65 and 72, I can definitely say it does hair better at 72. I know physics is NOT with me given that drag increases with the square of the velocity. Can engine efficiency at certain RPM overcome that drag?

As I said in another topic, I use the car only for commuting at it is 30 miles one way and majority of it is highway. Thus all my trips are pretty consistent.

What I have noticed recently is that my mileage definitely does not go down when I use the left lane more often which I did not expect.

I know few of you have made similar claims in the past i.e. the vehicle giving better mileage a higher speed but I did not believe them. I now believe them.

 

[shadow7]

Sometimes I wonder if we get a little better MPG because we get to our destination faster. Less time the car is running...

 

[901Memphis]

Every vehicle will have a different sweet spot based upon drag, weight,gear ratios and final drive ratios. I wouldn't be surprised to see some vehicles getting better mileage at a certain speed than others.

 

[JHZR2]

Specific fuel consumption differences across the powerband can be a number of percent.

Drag increases to the third power as a function of speed, so it grows right quick. Any gains from more efficient combustion should be offset by increased load.

But I guess that's why they have gear ratios, perhaps there are some points where one can design to do better, lean out, and allow some steady driving.

 

[aquariuscsm]

I get the best highway mileage when I set the cruise control on 85mph.

 

[johnachak]

Could it be the changeover to Summer fuel blend?

 

[Stregone]

My old buick lesabre got a little over 35mpg at just over 70mph. At 55mph it was more like 25mpg.

Based on what I have heard from others with larger cars and what I have read and experienced, a larger heavier sedan with a V6 or bigger is going to get its peak mpg somewhere around 70mph.

It seems like so long as the car is decently aerodynamic, it hardly uses more gas to push the car to 70mph, and the extra ground covered more than makes up for the extra fuel spent.

My new car is a tiny little Chevy Spark. It peaks at about 35mph where it gets around 70mpg. My old buick would be getting 15mpg or so.

 

[Revving2Redline]

70 vs 65 idk if thats enough info. I live on the Flint Hills on I-70 and I actually do better at 78mph in my Gdi hyundai. I run PP or PU 5w30 and found that going a tad slow was okay, but the steep hills made the tranny downshift often and seemed counter productive to just going fast enough to have momentum to make the climb. Using Injen short ram intake made it easier as I have more TQ and HP to help. Many have used syn oils to help with mpg, but I found a efficient fresh tranny fluid does wonders too. If you can get a filter/ intake to help thats also good. You want to use a good dryflow filter and maybe a drycharger on top to prevent any dirt if possible. Using quality gas has merits too.

 

[Nick R]

It really depends. I just drove from ohio to NJ and back. On the way out I did an indicated 39mpg, but when I hand calculated it at fill up, it was actually 42mpg. The trip back was very similar. This was in my focus, doing 76, alternating with the A/C on or the windows down.

 

[KitaCam]

Perhaps at the faster speed the car rolls without much need to accelerate (e.g. up hills)....whereas the more the car drops below the "sweet spot", more gas is required to get it back up to that rolling speed....

 

[02SE]

If the engine is more efficient at the higher RPM (enough to overcome the increased parasitic friction) it will get better mileage.

Studying BSFC, BMEP vs RPM plots would help explain how this is possible.

 

[HTSS_TR]

I tested on a flat surface for more than 1 mile each on 2 cars, S2000 and E430, for several speeds: 40, 45, 50, 55 and 60. With cruise control on my Ultra Gauge showed the best gas mileage was at 40 MPH, for each 5 MPH faster the gas mileage went down by 2-3 MPG.

I think the best gas mileage on a flat surface is when engine is at the lowest RPM and transmission in top gear without engine lugging.

 

[Kuato]

Originally Posted By: KitaCam
Perhaps at the faster speed the car rolls without much need to accelerate (e.g. up hills)....whereas the more the car drops below the "sweet spot", more gas is required to get it back up to that rolling speed....


I think you're on to something there. In rolling hills it definitely takes less gas to crest the top when you are already moving fast.

I would be interested to see if OP gets the same results on a pancake flat drive.

 

[Garak]

Every vehicle will have a different sweet spot based upon drag, weight,gear ratios and final drive ratios./quote]
Yep, I had been reading much the same from some carmakers a while back. Usually, the old double nickel is a great way to save fuel, but we do have some vehicles with a different sweet spot.

However, as was already mentioned in the thread by JHZR2, drag quickly takes over. We might see a vehicle do better at 70 than 65, but if we widen that gap much more than that, we'd see things normalize in a hurry.

 

[JOD]

Originally Posted By: Vikas
Can engine efficiency at certain RPM overcome that drag?



Despite all you read on this thread, the answer is "it's incredibly unlikely".

Power requirements increase roughly with the cube of speed. Sure, there are differences in efficiency across the power band, but once you get over 40-50mph, there's now way those differences will be enough to overcome the additional power requirements unless something is wrong with your engine or transmission.

Either there are other variables at play, your measurement is imprecise, or there's something wrong with your car. Unlike the alternative universe that is BITOG, no one who accurately measures this stuff has ever discovered that increasing speeds over 45mph results in better fuel economy--ever. The key is the "accurately measuring" part, and it's not as easy as it sounds.

 

[JOD]

Originally Posted By: Garak

Every vehicle will have a different sweet spot based upon drag, weight,gear ratios and final drive ratios./quote]
Yep, I had been reading much the same from some carmakers a while back. Usually, the old double nickel is a great way to save fuel, but we do have some vehicles with a different sweet spot.

However, as was already mentioned in the thread by JHZR2, drag quickly takes over. We might see a vehicle do better at 70 than 65, but if we widen that gap much more than that, we'd see things normalize in a hurry.


Yeah, no question the sweet spot may be different--but not that different. Here's a chart of some really disparate vehicles and the curves are pretty similar (except for one). Regardless, they ALL slope down after 45mph:

 

[eljefino]

GM used to cheat and trigger a lean "highway mode" at a speed slightly above what the EPA certified them at. Maybe 56 mph?

Was back in the late 1980s when they used removable PROMs and of course OBD-I.

This lead to excess NOx and when they got busted they put the keibosh on that.

 

[supton]

Originally Posted By: HTSS_TR
I tested on a flat surface for more than 1 mile each on 2 cars, S2000 and E430, for several speeds: 40, 45, 50, 55 and 60. With cruise control on my Ultra Gauge showed the best gas mileage was at 40 MPH, for each 5 MPH faster the gas mileage went down by 2-3 MPG.

I think the best gas mileage on a flat surface is when engine is at the lowest RPM and transmission in top gear without engine lugging.


Where I live my Toyotas would do double downshifts on hills if I were to try to cruise at 45 in top gears. My Jetta would fair no different. Someday I would like something to measure grade, we have lots of short but steep hills.

My truck won't go into high gear until at least 50. That would be 1,250 rpm. It has a tough time holding there at 55 mph and even 60. Not sure how much is lost from downshifting. I do have a hypothesis that I lose mpg when running with the TC unlocked, I figure, heat in the fluid must mean waste, so I force a downshift. Often rpm winds up almost the same so I think I win when I do this, albeit at the cost of additional Trans wear.

 

[Vikas]

Originally Posted By: Kuato
Originally Posted By: KitaCam
Perhaps at the faster speed the car rolls without much need to accelerate (e.g. up hills)....whereas the more the car drops below the "sweet spot", more gas is required to get it back up to that rolling speed....


I think you're on to something there. In rolling hills it definitely takes less gas to crest the top when you are already moving fast.

I would be interested to see if OP gets the same results on a pancake flat drive.


Given my driving, I tend to agree with this. I have set up the car computer to display instantaneous and tank mileage. I hand calculate tank mileage when I refill the tank and the number always match extremely closely.

When I am driving at 65, I am in the right lane. On my commute there are few rolling hills. I am reluctant to push the pedal down to maintain my speed unless somebody is right behind me. I can see how instantaneous consumption has adverse effect with the push of the gas pedal.

The only way I can drive in the left lane is by NOT watching that display. What surprised me is that I could do burst of 80+ and then let the pedal go and the tank mileage would recover as long as I had sufficient road way to coast back to my cruising speed. Depending upon the "tunes" playing in the car, I might be little aggressive (!) on some day vs others. I start with the tank mileage on the way to the work and my objective is to see if by the time I come back home, the number to remain same or increase slidghtly.

There could be other factors here, for example the traffic on the road. It might make driving at 65 needing to accelerate more often than driving at 73.

My other cars do not have this gauge, so I don't pay that much attention to the fuel mileage. I do notice quite a substantial difference between summer and winter mileage on all of my four car.

 

[Oldmoparguy1]

I saw something similar with my old 2000 GC. The thing I saw, was at 65 the 3.3 was only turning ~2200 rpm. It needed to be turning 2600 or so to get in the fat part of the torque curve. OTOH, my 2013 GC doesn't show that. That new 3.6 is just strong at any rpm. Amazing engine.

Another thing with the 2013 GC. Last July a 1900 mile trip, running 70 and up with the A/C on full, we averaged 25mpg for the trip with at least one tank at 28+mpg. This last Feb., ~same trip only averaged 23mpg. Was quite surprised. Winter Gas??

Wayne