I think you cannot use 6.5T on TA03. The gear ratio of the chassis itself is 2.XX. The spur gear is 80 and you should use motor gear less than 18 to fit the requirement. If not, your motor will become very hot. Also, the design of TA03 is not good for low T motor, the motor is covered by plastic and not easy to radiate heat. I think TA03 should not use too low T motor.
This is the general rule for using motor gear. For racing or running on track as other Chings had pointed out that it also depends on the traction of the track, the size of the track etc.