When I was running a 21.5 motor in VTA, the rule was that max (fastest) FDR allowed was a 4.2. At that time, I was running an 84 spur to a 50 pinion which resulted in 4.2 FDR exactly (84/50 x 2.5 gearbox ratio = 4.2). I should add that I was using 64 pitch gears.

If I recall correctly, if you are running a pinion larger than a 45 then you will have to cut out the chassis brace/webbing directly under the pinion gear. This is the area on the stock tub chassis that sits below the pinion gear when it is mounted in the car. You don't have to remove all of the webbing, just some of the "rib" that is directly under the pinion. You may also have to remove some of the rib to the right. Just remove enough of the rib to allow the largest pinion you plan to run to have good clearance.

It is possible to find smaller spur gears. I've run as small as a 80 tooth spur gear, but didn't like how close the motor was to the drive shaft when I wanted to run a smaller pinion (on tight indoor tracks).

My responses are assuming you are going to be running VTA with the rules set for 21.5 motors. In the US, the current rules for VTA are 25.5 motors with no gearing limit. I switched to a TC4 for VTA (same driveline as a TC3). With the 25.5 I'm currently running (82/53 x 2.5 = 3.86 FDR).

Hope this info helps. Let everyone know what rules/class you are running and we might be able to help with other setup issues.