>I ordered a Zuomini 2838 motor for my buggy and it has a 3.17mm shaft, which means I'll have to bore the pinion out, or special order a pinion with a 1/8" bore.

Yes I think I ordered about every 0.5mod pinion I could find.

AE has only the 16t 2.3mm 0.5m
There is a 15t i found on ebay for the 2.3mm shaft motors.

For the 1/8 shaft, you can get Hot Racing 0.5mod pinions 15t to 18t.

I don't have anything bigger than 18t in 0.5mod 3.175mm shaft (1/8), but I just ordered a few from China up to 22t. Not sure they will fit at the shaft.

Note - for the Hot Racing pinions (for 28xx motors) you will need to bore out the motor mount inner hole. Be careful. And as others have said, for 2838 motor you need to remove the small axle cover near the mount. For 2845 motor, you need to clean off the 2nd ESC tray mount in order for it to clear.

Final thought... with the spur gears available (58 being the lowest, and anything smaller is probably limited by the teeth-fit mount), the top speed will be limited by pinion. A 24T 48p won't fit inside the motor mount without modifying for clearance (and it wouldn't work anyway as it's not 0.5mod). If there is a workable 22T (which would be 12mm diameter) that would be the tallest gearing I'm aware of (and still not sure it would fit). With 65mm tires and ~50k RPM this will peak at ~55mph. So unless my math is wrong or someone figured out how to gear this taller let me know.

Probably not worth it because even stock they are quick enough for just about anything. But if it didn't break you didn't try hard.