Don't pick a gear ratio without considering tire size, a few inches in diameter makes a big difference on overall gear ratio.
Henry G's example above, while it is good advice if you have the same tire size he has, it could be wrong if you don't.
An example, at 60 MPH, same transmission, same gear ratio of 3.54.
One with 28" tires (like a 3/4 ton) will turn 2,549 rpm.
One with 38" tires (like a 2 ton) will turn 1,878 rpm.
Quite a difference, and with a 235 and 38" tires 3.54 gears would be a really bad choice. Then the chosen engine makes a difference, in my case 3.54 is just right with my diesel and 38" tires.
To decide gear ratio you first need to know several things:
RPM you want your engine to run, usually figured at cruising speed on the highway.
Transmission high gear ratio, for direct drive it is 1:1, overdrive, depending on transmission, can be between about 0.60 and 0.84:1.
The rear tires actual outside diameter.
Then with a gear calculator like on this page near the end
http://www.onlineconversion.com/bigger_tires.htm you can choose what rear axle ratio may be best for you. Or if you already have one, and want to pick a transmission, or tires, the same stuff applies.
Grigg
