The lowest possible FDR for the tt01 is 5.72.
This is when using spur 55 and pinion 25
(55 / 25 x 2.6 = 5.72)

Because of TT01's motor mount design, only a 55 / 58 / 61 spur can be used. and each spur size can only use certain pinion size as well. this is the limitation of the car.