[lincpimp]

I guess it has to do with load and motor construction. The higher end motors, neu, lehner, etc. seem to be able to handle higher rpm without a large loss in effecientcy. However if you atre looking to do high speed runs using tall gearing it will load the motor more. Using a motor that is too small for an specif application and revving the snot out of it to make power will not result well. From what I have experienced, keep the lower cost 540 size motors under 40k rpm and they will be fine. the 380 size motors can handle more rpm, but not more than 50k, and stay with light weight vehicle, 2wd preferably.