I've had a TC5, but run Xray now, and have recently purchased a 17.5 to test with. You need an FDR (final drive ratio) of approx 3.5 to 3.0 depending on track size.
Recently I tested my Xray with a 96-spur and 52-tooth pinion, for a 3.14 FDR. Since the TC5 has a higher interal ratio, you would need an 84-spur and 52-pinion to reach 3.23 FDR. The TC5 has a 2.0:1 interal ratio.
You might need to order some large pinions, since I've not found many LHS that have anything over 40-tooth on the pinions.