Hey Tom - I think I can help you out here.
First off, tires
- Carpet seems very popular with TRC plaid (jaco orange/purple) fronts and purples rear. If your not getting enough steering, than you can try double purple fronts. I use these combos on carpet with my xray and they seem to work very well. Tire traction compond can also be another tuning option. Full face rear and 1/2 inside front seem to be key. I use TQ tire traction compound, but another good brand is corally or paragon if allowed. I always use 28mm tires front/back but on the xray you can mount 30mm in the rear if you want more rear traction. Foam tires have less rotating mass than rubber, even at 28mm so don't be too concerened about that. Tire diameter is also key with carpet racing. I generally start around 61-60mm and ditch them when I hit 55-56mm (that is if they don't chunk first..LOL) Xray seems to really like the 57-60mm tires, but you'll have to adjust your gear ratio according to tire wear to keep the same rollout. That brings me to my next point.
First off you'll have to decide if you want to run 48 or 64 pitch gears. Both have thier advantages and disadvantages. 48 pitch is a little beefier of a gear. It has wider teeth but less than of them. (48 pitch means 48 teeth/1" diameter. 64 pitch is 64 teeth/1" diameter) So you see, 64 pitch has about 1/4 more teeth for the same diameter. Therefor it has more tuning options for gear ratios. (smaller increments per pinion sizes when matched with the same spur gear) 48 pitch is beefier and can be less succeptable to damage if you run outdoor asphalt racing where dust/rocks can collect.
Personally, I run 64 pitch since I like to have a lot of tuning adjustments and the gaps are not as large between ratios than with 48 pitch. I do know that a lot of team drivers do run 48 with great success so the choice really is up to you.
For 64 pitch, I only run 128 64P kimbrough spur gears, both indoor carpet and asphalt. The 128 gear is large, that means that you will have your motor lower for a lower center of gravity and if you got the Xray Factory Team kit, you have the 1.77 internal gear ratio, not the 2.125 that came standard with the T1, Evo 2 and T1R. The 1.77 gears with like a 120 or 124 spur gear will put the motor in contact with the motor can and the results is not as free of a drive train and also heat from the motor can degrade the belt (though kevlar lined). Thats why people only run the 128 spur gear and adjust their pinions to the ratio that they want to run. (you'll have to adjust the 128 gear to 48 pitch if you decide to run that way. Either a 93 or 94 spur gear I believe will be compatable)
What motor style are you going with lightspeed? Is it a monster based motor? If so, gear around a 7.5 with 60mm foam tires.
I hope this helps you, ask me if you have any questions or check out the xray thread. There is loads of info there.
Happy Racing and putting together your new Xray!