Gear Ratios

That's just it, if i was doing something completely different (robot wheelchair in this case) how do i figure up the final ratio, i really have no idea how to figure this. I would like a ratio of at least 7.14:1. What number of teeth on each sprocket would it take to get this in gearbox form (would be using 4 sprockets) because i'm limited on space and smaller sprockets are cheaper.