[lincpimp]

I would go with a 12 or 13t pinion for your setup. Your motor speed is a little slower than optimum, and 16v is a bit optimistic to expect from a 4s pack under load, I usually stay with the nominal voltage for lipos, as it gives a better indication of voltage under load. I do not think you will have any issues with the vehicle geared for 38mph or so. I would use the 13t, but you may want to go down to the 12t.