Didn't I say that in Post #39? The Final Drive Ratio (FDR) is much more important then the gear ratio. The FDR is readily changed without removing the motor or gears, just change the tire diameter. The FDR also changes during the race due to tire wear and this must be taken into account before you decide on beginning tire size.
Rule of thumb: .020" = one tooth.
IOW, regarding FDR, a change in tire diameter of .020" is roughly the equivalent of a one tooth change of the spur gear.
To be exact, when using a 10t pinion, if you change to tires that are .02632" bigger in diameter, the FDR will be the same as staying with the original tires and using a one tooth smaller spur gear.
However, when the pinion is 14t, .01587" tire diam change = 1 tooth spur change.
The average is about .021" (approx. rule of thumb).
If you don't know, but are curious as to what Final Drive Ratio (aka rollout) is, it's the ratio of armature revolutions per 3.14" of car travel.
It's easy to find rollout charts but here's my version:
Gearing - Final Ratios ver.06.pdf 170.5KB
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