One thing that has changed with 12th scale (and RC in general) is that people no longer focus on a specific gear ratio. Everybody now does it with rollout/FDRs which take into account tire size. For 17.5 you will want to start around a 100mm rollout (which means the car will travel 100mm per rotation of the motor). Things that go into the calculation are:

- Tire diameter
- Pinion size
- Spur size
- Transmission ratio (in a pan car this is 1:1)

The calculation is:

(Tire size * pinion * pi) / (spur * internal ratio)

So if you are running 41mm rear tires, a 72T spur and a 56T pinion it works out as follows:

41 x 56 x 3.1416 = 7213.1136 / 72 = 100.18mm rollout.

As tires wear the diameter will get smaller, for example using the scenario above but with 40mm tires your rollout is 97.74mm so you would want to go up 1-2 teeth to get back to around 100 (1 tooth would be a 99mm and 2 teeth would be a 101mm).

As for what rollout you want to run this can only come from experimenting. Start around 100mm and check temps after 8 minutes. If the motor is hotter than 140-150 then decrease rollout a little at a time until temps are within reason. If it is cooler then increase rollout until you get too hot. I generally do 2mm jumps either way when I am tuning.

http://www.rctech.net/forum/electric...e-rollout.html

EDIT: Attached are the 2 charts I made in Excel. I run a 72T spur in 17.5 and a 76T in 13.5 currently. The green is my target rollout which I specified in my spreadsheet (100mm for 17.5 and 90mm for 13.5). Feel free to use it as a guideline.