A 106/53 combo in an XRay T3 and T4 comes out to a 3.8fdr (106/53x1.9). I posted that as an example of how I run the largest spur/pinoin combos, while still achieving the desired ratio. A rule of thumb with the XRay's 1.9 internal ratio is that for every 2 teeth you go up, or down in spur size, you can match it with a 1 tooth change on pinion and achieve the same ratio.

106/53
104/52
100/50
96/48
92/46

All of those combos will net a 3.8fdr. I normally run a 3.6 in my VTA, but went up because I'm racing at a smaller track this weekend.

I'll go with a 96/55 combo (3.32fdr) and see where it gets me next week at Thunder R/C