Highway Gas Mileage

[euromodder]

Originally Posted by Old Mechanic

I get the same mileage at 55 on a deserted US Route 60 as I do at 65-70 on the packed I64

As SentraSE-R said :

You can't fight the laws of physics.

There's more drag at 65-70 than at 55 - be it aero, rolling, or mechanical. No way around that.
The question is how the car copes with that.

Old Mech's Fiesta will be in the same gear at either speed - so it can only be a matter of gearing and engine efficiency at the corresponding rpm.
Both are choices that Ford made when bringing it to the market, and since they don't cater for hypermilers, the Fiesta is likely geared to be at its optimum rpm somewhere around those higher, more normal speeds.

Lower rpm is only OK if the engine can still pull it off OK.
It's not always or necessarily better - though you always win a bit by not having to accelerate the extra 10-15 mph.

Originally Posted by basjoos

But you can modify them with effective aeromods to flatten out that drop in mpg with increasing speed

Sure you can mod the hell out of a vehicle and reduce its drag, but it's still there - and increasing with speed.

From the speed / MPG numbers you've posted before, I recall that if you drive slower, you also get (even) better MPG.

[euromodder]

Originally Posted by skyking

It does go back quite a ways, that beneficial effect. I wonder if a technically savvy guy could make a Cruise control input coupled with a laser rangefinder.
Maybe some indicator lights.

Two dots or thin vertical lines on the windshield could help one to keep the ideal distance - dambuster style - though not automatically.

The width of semis is a known constant.

[skyking]

Put on your drafting glasses!

[Fat Charlie]

Originally Posted by skyking

Put on your drafting glasses!

No, get a drafting hood ornament:


Don't worry about any aero penalty, the semi will handle that.