The engine isn't really important...

I am running a torky Pontiac motor... 389 with a 1/2 inch stroker crank. 447 inches total with a 5 speed od transmission.

Some interesting engine facts for my application a 59 gmc 1/2 ton longbed.

When I got the truck it got 10 mpg with 3.9 gears, a 336 v8 and sm420 transmission. (28 inch tires)

I quickly changed the transmission to a 5 speed with .73 OD ratio and changed the rear gears to 3.38:1. Mileage increased to about 12 mpg. (still using 28 inch tires) Engine noise was significantly reduced at highway speeds.

336 v8 got tired so replaced with current motor 1959 pontiac bored and stroked to 447 inches. New motor destroyed the 3.38 rear in less than 30 miles. Replaced rear with a 3.70:1 ford 9 inch rear. 14 mpg (still using 28 inch tires)

changed rims and tires to 31X10.5 tires. Mileage increased to 14.5 mpg. Performance was sluggish in all gears. The taller tires effectively changed the 3.70 ratio to 3.34 ratio. 4.11 gears would cancel out the taller tires putting the truck back to where it was with 28 inch tires and 3.70 gears.

I then changed the rear ratio to 4.30:1 instead of 4.11 gears. The difference is about 100 rpm at 70 mph. Mileage decreased significantly to about 12 mpg.

Why such a drastic loss of mpg? The higher profile of the truck with the taller 31 inch tires actually creates more of a drag at highway speeds. The drag is caused by the air movement under the truck.

Taller tires will lessen rpm at freeway speed, but can also rob mileage by increased rolling resistance and cause other drag variables.

Anyway... look at new cars or trucks and compare what gear ratios are being ran with tire size. That will likely be around the guidelines above. Again... the numbers can be changed a bit. taller tires or higher gears can be substituted to move towards economy or power.

My pontiac likes lower rpm around 2200 for cruising. Your sbc might actually like higher or lower rpm. I think cruising at 65 mph, rpm shouldn't be lower than 1800 or higher than 2600. Again those numbers aren't set in stone and don't take every possible variable into consideration. A very light vehicle might be happy at 1600 rpm.

My mileage experience is based on carbuerated vehicles.

Last edited by bigbadswingdaddy; 02/02/2013 11:06 PM.