if you are talking about foam tires with the GT12F then you need a roll out number rather than just pinion and spur as the final drive will change with diameter of the tire as the foam depending on where the diameter you start it at and as it wears. Also the gearing depends greatly on your motor and track size. The larger and more open flowing the track then you can gear a little higher but if is short tight and technical then you will want a lower gear ratio/rollout to help you get up to speed quicker.

For me, I use primarily Motiv MC4 motors on a 120x50 I want to say track. If its a flowing layout I have run up to a 103-104 rollout and finished a 6 minute run at 165 deg F. On our current layout which is a little more technical I have been messing around with my motors (timing and brand) and changing things around to get the best for an upcoming race so I have been anywhere from 90 to 98 for rollout and coming off the track between 140 and 170 degrees. I try not to stay in the 170 range but it did happen a couple times while I was trying things. To get to the rollouts above I generally run a spur in the mid 60's to low 70's depending on what rollout I am trying to get to and what diameter the tires were at.

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