I'm going to side with SlowerOne.
I've been told from the old time oval racers that keeping the roll out the same, go the largest spur/pinon combo you can cram in there. Most oval cars are pan cars so using a larger pinion/spur move the motor closer to the pivot point making the reducing the unsprung weight for better reaction to bumps.
I was also told that gears under 13 teeth get very inefficient at transfering power compared to larger gears. What SlowerOne pointed out that the "flank" or face in contract. This is critical for transfering torque. Since motors produce power in phases, the lower tooth count gears may stall a little bit before turning if the torque is needed between a motor's phase. The higher tooth count helps reducing the cogging that can happen and help the gear "ratchet" each degree forward more easily. Yes inertia does keep the wheels spinning duning the dead band in the power phases.