I'm currently running a 14/42 front/back combo. I can barely get to 5th by the timing gate and it flattens out around 6,800 rpm and 90 mph. I think I need to run in 4th. Mathematically I can reach the desired speed (90-100 at 7,000-8,000 rpm) with a 16/40 combo. It is taller gearing and assumes 4th can pull it.