Internal ratio of the tt01 is 2.60. If you have a 61t spur and a 21t pinion that gives you 7.55 Final ratio ( 61 divided by 21 =2.90 x 2.60=fdr) Thats geared to the moon for a 17.5 !!!! The problem with the TT01 is that you can only use certain size pinions. The motor mount has specific mounting points for each size pinion you use as the mount is not adjustable.
I don't know that you can get the gearing to an appropriate number with that car. In our club we run pretty small , very technical track layouts and a 5.0 fdr is good. For a larger track we had to gear in the low 3's.