Reset the mileage on the freeway. 10 miles on relatively level terrain gives me 40.4 mpg with Cruise set to 50 MPH. Slow and easy = impressive mileage. Wish it would hold that in city driving.

 

Many people assume that mileage is linear, i.e., doubling the speed drops the mileage in half.

But mileage is exponential. If you have ever seen a graph of fuel mileage plotted against speed, above 45 mph, fuel mileage actually drops off like a rock.

So increasing speed from 45 mph to 60 mph can cut fuel mileage in half. Increasing from 60 mph to 75 mph can cut fuel mileage in half again. So 40 MPGs at 45 mph can drop to 20 MPGs at 60 mph and to 10 MPGs at 75 mph.

Percentage decrease, of course, depends on the vehicle and its coefficient of friction. But as these trucks are essentially shaped like bricks, the downward plunge is particularly steep.

That is why I find it particularly amusing when people post about poor fuel economy. "How fast were you driving?". "Only 75-80 mph!".

 

Yep. For turbulent flow (which happens with our trucks) drag is proportional to velocity squared.

Mileage is pretty flat from 25 - 60 mph, dropping off pretty quickly above that.

 

The graphs of MPGs versus MPH that I have see show a fairly steep drop starting at about 30-45 MPH.

This is for a Chevy Cruse.

http://www.cruzetalk.com/forum/attac...zempgvsmph.gif

 

40 mpg with an Expedition in any circumstance, if you're not going downhill with a tailwind, is pretty amazing!!!

I agree mileage is exponential and with the large frontal area of an Expy you're gonna get way lower mileage at 70...and I have a hard time going slower on freeways than 70-75 because that is the speed at which traffic moves. My experience is also that mpg is best at maybe 35-40 mph, drops a bit as you approach 50, and drops noticeably for each 5 mph over that....

If you can get some full-tank mpg numbers, I'd be interested in seeing them just because...

 

I do full tanks of 22-24 if I am careful. That is running in 60% highway, but at speeds no more than 55 mph. Not too hard to do here because we move pretty slow here in the Deep South. As I tell my Northern buddies, those who moved faster didn't live to reproduce. :-)

But if I am not careful I'll run around 20-22. In fact I have about 10,000 miles on her now and the total average is 20 mpg.

 

Great average mpg, FreeWill.

Over 130k miles, my '02 E150 (with a 4.6) averaged right around 16 mpg. (Yes I tracked every tank of fuel...) I could nurse it over 20 mpg on the road if I went 50-60 mph. You've got a lot more power, more drivetrain drag (assuming you have 4WD), wider tires, little less frontal area, similar weight, but a couple more gears in your trans. If I drove 55-60 in Michigan, I would be killed by a semi rear-ending me at 20 mph faster than I'm moving, even in the right lane...

It's not speed that kills so much as it is a difference in speed

George

 

2WD on my Limited. Never need 4WD. And here I just hang in the slow lane, relaxed with cruise control saving me gas.