Ken, here are a couple of links that might help.
Speed for RPM Range - This formula will calculate the speed of the vehicle at a RPM. You enter your tire diameter and your rear end gear ratio and it will display vehicle speed an RPM range from 1000 - 8000 RPM.
Speed for RPM range comparing rear gears - This calculation will display two tables with mph vs rpm range for two different gear ratios. It's handy for comparing different speeds for two different gear ratios. In other words how will changing my stock 2.73 gears to 4.11's effect my cruising speed. Same as Speed for RPM range below except with an additional gear ratio input.
And here's one more to help calculate tire dimensions:
Tire Diameter This formula will calculate the tire diameter from the tread width, series, and wheel diameter. In other words the stamping on the tire, example 275 60R15.