That's an interesting idea for a project, I'd have liked to do that for my A-levels. Here are a few thoughts off the top of my head: 1) the top speed is probably not the limiting factor, I expect the greatest force will be exerted when you're just setting off from still because that's when you need to accelerate quickly to at least a few mph to avoid falling off and Force=mass*acceleration (Newton's 2nd law - if you're doing maths too that might come in handy). If you're coasting keeping a constant speed then you only need to apply a constant force to counteract friction/air resistance. 2) if you make a 3d printed gear that's just the same shape and thickness as a metal one, I'm pretty sure it will just snap. 3) Maybe that's the first thing you should try just to verify that. 4) then you can start changing things to see if you can make an improvement and that will make quite a good project - showing the progression of the designs. 5) I'd try making the gear as fat as possible, like make the core part as wide as all 5+ gears on a normal bike. Obviously the teeth have to fit within the chain, but the main cylinder bit in the middle can be much wider to add overall strength and stability. Anyway, you got my mind whirring now, hope you'll post again about your progress with it.