Something to keep in mind when changing gears is the "pitch" of the gears. If the pitch of the gears are different, then the gears won't mesh properly. The pitch defines the size and spacing of the teeth. When I raced 1/24 scale commercial track type cars, 48 and 64 were the two most common gear pitches. It is not possible to use a 48 pitch pinion with a 64 pitch spur. The size and spacing of the teeth are different. You could get the gears to mesh at one point but once you tried to turn them they would bind up.Also, the number of teeth defines the diameter of the gear, assuming the pitch stays the same. It would not be possible to have 48 pitch pinions with 8, 9 or 10 teeth that are the same diameter for example. It would require changing the actual size of the teeth to do this.