Welcome to the club Roy. The best advice I can give with the gear ratio is to download an app for either Iphone or Android. I use RC Final GearRatio on android and it is free.
The gear ratio will depend on the track you are running on and your style of driving.
I run 72/32 with a 10.5T. This ratio with the 13.5T will give you a final gear of 4.5 and a 41.87 rollout potentially 73.9kmh (45.94mph) this assumes 60mm wheel size and that your Kv is 3500 on your 13.5T