there is a whole lot more that comes into play, like how much power do you have,you could gear the tractor to go that speed and not have the power to pull that gear and give acceptable acceleration or if your tractor is real heavy or you are real heavy you may need more power, and you may not need to even go that fast to be the fastest one out there. we have beaten many guys with more power than us by simply having our tractor handle better and we could get on the throttle sooner or be on the throttle longer into the corner. the gear ratio is also dependant on what rpm you want to run. so I say just put some gear on there and go out and get some seat time and see what you need from there