It's quite simple choosing what final drive you require.

Choose your realistic maximum top speed, wheel size and maximum engine rpm. From that work out your final drive required and choose the one thats nearest.

Our ERST top speed record with Oli's car was done on std gearing with 7600rpm limit for 174mph. Obviously if the engine did'nt rev as high we would have to have used a longer final drive.

Just do the maths and work it out, don't guess!