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.

 

[Frank]

You'll lose another 15-20% through the driveline,

In the trucking business we used 18% for this calculation, powering two differentials.

tooling along at about 60 MPH in 5th with engine speed at about 1800 RPM and this seemed to be my car’s happy place. Not a lot of road noise; not a lot of engine noise and not a lot of wind noise. Then I bumped up to about 70 MPH and now I’m at about 2000 RPM. The car was happy at that speed but the road noise had increased noticeably (no blanket on the firewall). The most noticeable thing though was the wind noise; the increase was really noticeable.

60mph-70mph= 36% wind force increase. Noise is created by turbulent fluid flow. Just like porting a head, flow dynamics change with velocity and force. Air that can make the turn around a corner at X speed may break into turbulence at X plus more velocity/force. So we have a 36% increase in force on the non-aero windshield with a 16% velocity increase. A proportional increase in turbulence is proved by the increased noise of the tumbling air. Akin to water waves breaking.

Interestingly, the ideal aero shape is not a long triangular wedge or an egg. Ideal aero is the same as the a water drop moving into the air with the fat bottom side leading- not the tapered upper side leading. A cabover semi tractor has only 2% more aero drag than a Conventional tractor. If someone had rounded the sides of the cabover only a few degrees, it would be the best aero on the road.