Hey Mac,
The gearing you have is one step away from the lowest drive combination of TT-01 spur and pinion gears. If you use the 55T, that will be the fastest gearing with the gears intended for the TT-01.
It's hard to gauge whether you are over-loading your motor in terms of temperature if you don't have a temp gun. However, I think something around a 22-25T with your spur should be fine.
I think some motors tell you the recommended FDR for the car...does this have one?