Here is the calculator I like.
http://www.drivetraindirect.com/t_gear_calculator.htm I use it to find rear axle and overdrive ratios that work together to give me the cruising down the interstate speed I want at an RPM that gives the engine the best economy and a long life.
If you find a transmission that works well with the rear ratio you have you can still have the lower gears needed to race from light to light, and the top gear for good fuel mileage.
Grigg
