Thanks for your help everyone. The main goal we are looking for is top speed. If the ratios are the same from 1st to 4th in both boxes their isn't really an advantage by switching to a fivespeed? What would be the best way to get the most drive out of the car? Gearboxes we have tried are 4.3 4.1 and 3.9 we have always used a four speed gearbox and most generally run 2nd gear with a 3.9 however we feel that we could get even more drive out of the car. Thoughts anyone?