I found the following tip at Trinity's site:
9) What gearing should I start with on my Sedan?
(Drop one tooth on the pinion for every turn you drop on the mod motors)
Losi XXXS 48 pitch (Stock - 25/90) (10T Mod - 19/90)
Assoc. TC3 48 pitch (Stock - 26/72) (10T Mod - 21/72)
Schumacher Mission 48p (20t pulley) (Stock - 23/90) (10T Mod - 19/90)
Yokomo MR4 48pitch (Stock - 26/81) (10T Mod - 20/81)
Hudy X-Ray 48 pitch (Stock - 27/93) (10T Mod - 22/93)