I said someone would be along to correct me! LOL
I run a 104 tooth spur, and the standard drive pulley with a 5.5 brushless geared on a 18T pinion. I'm usually too lazy to change this.
I don't know a huge amount about gearing, but this works for me in most situations, but can get hot on tight indoor tracks.
One final piece of advice from me is to use plastics rather than graphite wherever you can. Plastic tends to bend and give a little before breaking, whereas graphite will break under similar stresses. I can't really tell the difference in feel between using graphite or plastic wishbones for example, so why go with something that breaks more easily is the way I see things.
Bear in mind though, you can no longer get plastic rear wishbones - they've been discontinued by Losi.
Why don't x-factory produce their own parts by the way Chris / Dan? Why do we have to rely on Losi for shock towers and arms?