GADad,
To run the numbers take (MPH x Rear gear x 336 x OD ratio)/ Tire Diameter Inches to give you cruise RPM. For example, at (70mph x 3.00 x 336 x 0.73)/30" yields 1717 RPM. The "336" is a unit constant, dealing with differences of miles to feet, hours to minutes & diameter to circumference. You want your cruise rpm to be at/near the bottom of the torque band on your engine so that you're not having to shift down to 4th every time you get to a hill, or even to pass on the interstate without flooring it. If you get your cruise rpm too low your mileage will actually suffer on all but near level terrain, but your top end will be awesome!