As the tire size gets smaller (foams), which way should I go with the pinion? More or less teeth? I got about 50 minutes on my tires and I noticed i was really getting beat on the low end the last few runs, top end was only a little slower. I can see my car is really dirty so I know the foams have gotten smaller. I run a JRX-S with CO27 motor. Tires started at 63mm and I was runing 21/96 with great results. I run a a freshly paved blacktop track if that matters.
A larger pinion will spin the tires faster and make up for their smaller diameter. The change in circumferance will determine the ratio change. Circumferance is diameter x 3.14. Going up to a 22 pinion will be a 4.7% change. Did the diameter of the tire change that much?