Another fuel economy check with my '65 one ton

[66Submarine]

Originally Posted by factorystock

Rear axle ratio on '62 C 20 was 4.57, there were no other optional ratios in '62. Engine condition is also a factor in MPG. A worn 235 will not get the same mileage as a healthy 235 or a worn 250 will not get the same mileage as a healthy 250. A speedo is not accurate once the original diameter tires are removed. The formula for RPM: RPM= MPH x axle ratio x 336 divide by tire diameter. Example typical C20 at 60 mph ( a top speed which I would be comfortable with: 60 mph x 4.57 axle x 336 constant divided by 32" tire diameter = 2879 rpms . With a 4.11 pumpkin change 2583 rpms.

A 32" tire is also not actually 32" for the purpose of figuring speed (etc.); you need to use the loaded radius/revs per mile.

My 32" tires take 650 revolutions (or so) to go a mile, which would make them 31" if you were using diameter to figure speed. Using 32" gets you 630 revs per mile.