And in IMOW, you will race in the highest gear available. If you have a 5 speed, you will race in 5th, not 3rd. You just have to gear the mower to 8 to 1 using pulleys and gears on the transmission and on the axle.
Also, where are you located? There may be racers very close by you that can will let you go to their house and look at their mowers and give you suggestions on how to build.