So I've raced go karts for years and the gear theory there has always been to run the biggest drive gear (pinion gear) possible. Then match your spur gear from there to get the final drive ratio you want. I just wanted to know if that's the same in RC. In addition to that I have a b4.1 with a 17.5 on a pretty tight track and i don't know where to start on gearing. Help?

im not sure but i can say that the xray 808 had large drive gears (clutch bells) and smaller spurs...like 16 or 17T with a 41 or 42 if i remember correctly....arguably this was not ideal in terms of what the pro guys required i guess....so with the xb9..their latest buggy..they went back to a more conventional gearing found on alot of newer cars..ie..a 13T bell and a larger spur gear...im not 100% sure what the sixe on the xb9 spur is but for on the mugens most guys run 44T to 48T. i guess like all things it comes down to preference, engines and track conditions probably

B4.1 is the 1/10 electric buggy. Assuming you have a good lipo batt. The general rule with brush less all comes back to heat. Shove the biggest pinion you can on most likely with stock spur 29-30 tooth pinion and run hard to 5 min. Simulating racing conditions and temp it. As long as you are below 140 degrees id say run it and have fun.

^+1. Heat is the biggest determining factor in gearing for electric racing (considering the performance is on par with expectations). But you'll also want to take in to consideration timing (end-can and ESC). Both can make a sizable difference in your ultimate gearing. Start around 26-30 with timing, and 30+ without and see where the temps fall. If they are high drop a tooth or two, if they are low add a tooth or two.

My B4 baselines
-75/32 no timing
-75/26 with timing

Just note, when opening thread about electric racing it may be best to open in the electric racing portion of the forum. It helps you to get your question seen by the right people.