In the dark ages, when I raced karts, changing one tooth on the axle could make a difference in lap times. At one race, by subtracting two teeth from the axle, I went from third place in the first heat to first Place in the next two beats, by a bunch. It never occurred to me to change drivers. With your gear ratio, one tooth on the driver is equal to four teeth on the axle. Now I understand that your racing surface is not stable, and changes a lot, from the start of the day to the end of the day, and for the day, four teeth might be appropriate, but I would change the axle before changing the driver. Just a thought.