[MetalMan]

If you want to hit 55mph, you need A LOT of space and good radio range. Back when my BL controllers weren't burnt up, I was using 4s Lipo (14.8v) on a Feigao 12s motor (so at full throttle the motor was spinning ~40,000RPM). I did many speed runs, and in a space of about 200ft., I got to 56.6mph (this distance included braking distance), and was GPS tested. So, if you want your Rustler to go really fast, you need a huge open area to it in where your radio won't glitch.

With the 8s, you should not exceed 9 cells for bashing. For better efficiency, 8 cells would be best. Whenever you change cell counts, you should change gearing to keep the motor in its good temperature range. To find the best gearing, it is best to just experiment, as differences from Rustler to Rustler can make gearing of one motor could make another motor heat up too much.