[jhautz]

50mph may be a bit much to shoot for with the 1512, and I prefer a system that doesnt quite run at that rpm, but that said... I run my revo on a 1512/2.5d and 5s and it has no problem geared for 42mph and run on a track. In race conditions it comes off the track after 15 minutes solid run at a cool 140F. I have run a 1515/2.5d in the same setup and it came off around the same temps, but I had it geared a few mph faster. With the setup I have I think there is still some headroom to go up in gearing, I just dont need it for the track, but you should be able to squeeze a bit more from the motor than I am now.

I think you can make that motor work, just start on the conservative side (around 35-40 as others have said) and see how the temps do. if they are fine add a tooth or 2 untill you find the point where you start to run into the motor limit. I dont think you will be too disapointed. Those 1512s can put out some power.