Using one of the calculators at Grigg's link (above) here are some calculations regarding rear-end-ratio, tire diameter (loaded), MPH, and resultant RPM:
4.56 rear end; 1:1 top gear ratio; 65 MPH; 30" tires: 3,300 RPM
4.56 rear end; 1:1 top gear ratio; 70 MPH; 30" tires: 3,575 RPM
4.56 rear end; 1:1 top gear ratio; 65 MPH; 28" tires: 3,557 RPM
4.56 rear end; 1:1 top gear ratio; 70 MPH; 28" tires: 3,830 RPM
Just for the heck of it, let's assume you have a larger diameter tire and ran at 80MPH:
4.56 rear end; 1:1 top gear ratio; 80 MPH; 32" tires: 3,830 RPM
Sure, a 235/261 could run at 3,800 rpm, but, for how long? Maybe someone has/had a speedometer that needed calibration, or very wide diameter tires? I think a high-pressure 235/261 would be hard-pressed to run all day at much over 3,200 RPM.
Around 2,800 RPMs is what I got at 65 mph in a 1/2 ton 54 Suburban with stock 3.90 rear-end and about 31" radial truck tires. I got 18 mpg on 350 mile trips at 60-65 mph. A switch to 3.55 rear gears brings it down to 2,500 RPM.
Tim
