clowe1965

Its a lot more than the speed you are traveling. Ideally, you would try to make the car go as fast as possible using the least amount of fuel (Duh). This is done by matching the rpm that the engine has maximum fuel efficiency, the output of the engine at that rpm and throttle position, with the power necessary to overcome the highest drag force possible. The last part is done with the gear ratios. Example would be ( i did this a few year ago) a truck with a drag coefficient of .5 (prius has a Cd of .2-.3) requires approximately 50 hp to maintain a speed of 60 mph. Formula is 1/2*rho*Cd*Area*Velocity^2 for drag force, drag force * Velocity is power (in watts), then convert to hp if anyone cares to figure it out.

End result is probably around that magical 55 number because engines are only 18-20% efficient (avg) with most of the energy going to heat, and the drag on cars, while improved over the years, is still there.

Wpipes

Thank you for the elaboration . My brother explains this to me everytime he's in the car with me and I go over 55mph (he is an engineer). Bunch of wisenheimers if you ask me (I kid ).

JIM5.0

With a 3.31:1 rear gear and stock sized tires, I actually notice my best avg MPGs increase above 60MPH in 6th gear (OD).
I find my peak avg MPG to be at 70MPH given no wind, straight flat highway with light traffic where everyone is going speed limit (70MPG PSL)

Now, with higher rear gear ratios on an MT82, like the 3.73:1 rear gears, yeah, you might be getting your peak avg MPG at around 60MPH

clowe1965

Wisenheimer indeed, it is my duty as an engineer to educate the masses! I miss my university paid for software that let me play around with equations like that. I wish we had an instantaneous mpg meter, would settle a lot of speculation, assuming we take everyone's word on "level ground" and "no wind". No offense intended Jim.

JIM5.0

None taken at all, and I am actually glad there are ppl who understand this sort of stuff, even if we are the only ones that do.
I unfortunately do not have access to such software, so in my case, I would have to calculate it the hard way: calculator and graphing paper.

The only measuring tool I used is the on-dash MPG calculator. I typically dont use it in an instantaneous manner, but on the few times my curiosity hits me, I hit reset while cruising down the freeway to see what my instantaneous MPG is.
I don't know just how accurate this bit of software is in our cars, but since it is measuring the amount of fuel being burned in comparison to speed, I assume it works well in guessing just how much fuel you would burn in a mile if you remain at those conditions.

Others have expressed that the on-board MPG calculators are not accurate, others have said they suck at instantaneous measurements and are only useful over very long times, like over a weeks time or even a months time, so I am actually up in the air on this one on what to believe

clowe1965

I no longer have access so the calculator and paper is my method now, just reminiscing. Considering the device in question and its data recording interval I do not have much faith in the dash meter. Unless they actually put a calibrated flow meter in the fuel line (doubtful), the gal/min measurement is probably from an empirical flow curve that is calibrated to the amount of amps the pump is drawing. It could also be something that is pulled off the fuel maps and multiplied by the correlating rpm. It would be nice to know what the exact mechanism is, and short of direct explanation from a Ford engineer/designer we will never know. Same kind of thing with the speedometer not matching the mph on some gps units. If those two different systems dont match up, who is to say that the mpg is right either?

mph07alloy

I've got 11000 miles now on my '11 with the 3.73 rear. On a recent road trip of 1200 miles my highest mpg was 23.58 on the highway between fill ups (using a gas mileage calculator app on the phone, not the one on the dash) This is averaging between 80-85 mph. City driving fluctuates, I see between 13 and 17. As someone commented before me, the MPG did increase as the miles went up. I was getting like 12-13 city and 20 highway before the 5k mark.

jakegoku

I see between 19 and 23 mpg on regular fuel in my GT

jakegoku

Hey guys,

I'm a mechanical engineer, so I've been wondering about "hybriding" the Mustang somehow for fun even if it's a conceptual design.

Have any ideas? Crazy ideas are fine even.

Mine:

1. traditional battery and motor
2. flywheel
3. heat salvaging
4. harnessing vibrations from suspension

Falcon Fixer

Just drove the 650 miles back from the Super Bowl.
I-70 doing 80 mph the whole way.
24.6 mpg with three people in the car.