OK, here's the scoop for gearing the Tamiya TB-02 for USVTA 21.5. I geared to 4.37 to start out, and the gear mesh is perfect, so I'm assuming that the other combos will work as well as the methodology to calculate them was the same.
All of the following numbers pertain to 64p gears (use the "hockey puck" spur adapter that came with the kit).
FDR 4.37:
- 74t spur, 44t pinion, motor mount position B.
- I got these from superiorhobbies.com. 44t pinion=RRP4344, 74t spur=XENG640074
- tested and meshes perfectly
FDR 4.28:
- 74t spur, 45t pinion, motor mount position C.
FDR 4.25:
- 72t spur, 44t pinion, motor mount position A.
- 72t spur may be hard to find, but PRS just started making them.
FDR 4.47:
- 74t spur, 43t pinion, motor mount position A/B.
- 3racing mount A/B position will probably work perfectly with this combo
FDR 4.58:
- 74t spur, 42t pinion, motor mount position A.
It wouldn't hurt to use the 3racing motor mount with any of these combos as it gives you finer mesh adjustment.