For speed and big engine drivers only - yep I know don’t run .21 engines in HPI cars - some drivers do - and ask for more - the bell  besides the R40 gears can use al HPI RS4-3 screw on gears , second gear from the RS-4-3 has to be used as first gear -- I have a very limited supply of >23 >24 >25 from the MT challenge- Those gears go to test drivers first ----Please don’t ask I have drivers -- if it works out I have gears made--- those gears are for testing  only .  Gear set up-- bell 22/25 and spurs 44/41 would be the biggest spurs  possible with out doing chassi work . At 30000rpm ----? 4 inch tire theoretical speed about 75 mph and as we all know speed cost money -- so please no lectures .