All Small Six C4 Gear Vendor overdrive

[beanmiester]

So the fastback is fastest in reverse got it

 

[rickwrench]

Kinetic energy = 1/2 MV squared. The effect of increasing speed is far more energy-intensive than our human logic casually assumes.
More engine power is required to move faster, regardless of RPM.
There is 2X more drag at 70 mph from 50 mph.

Using handy "HP required for speed" calculators. You can get pretty close to the max potential mpg possible at each speed.
I've used rough estimates, converting to watts, for calculator-easy numbers.
Gasoline has about 33,700watthours of power. Carbed engine, tuned well, is going give you about 1/3 of that, about 11,000wh. You'll lose another 15-20% through the driveline, automatics usually skewing to the worse side of that range. Call it 9300wh per gallon of rear wheel power, or, close enough. And, one (-1-) horsepower = 745 watts.

Plug your numbers into one of the many online the hp/mph calculators, your cd (about 0.45 for my Falcon), frontal area (22 for me), weight, and speed. It'll spit out a HP number. I get about 25hp for 65mph:

65mph: 25hp x 745 is 18,625 watts, per hour. /9,300 = 2 (gallons). 65 miles travelled in one hour/2g = 32.5mpg @ 65mph.

With a well tuned Holley 1946 on my 200, DSII, T5, properly inflated 185/80-13s, this is right on the real world mpg number I get on the highway @65mph. That's the speed I drive on the highway in the old Squire. 64-65mph.