you can use a 5.5 540 but for me the 6.5 lrp with a 13mm ht rotor geared at 16/93 was stupid fast so I don't see the need. I was using a lrp sxx comp v2 speedo and yes it got hot motor and speedo but I put a fan on the motor and speedo and atleast for the main it wasn't to bad. The only time I lost with the 540 can motor was when I had a motor break on me in the main and the other time my charger f'ed up and didn't charge my battery and it said it did and I hit lipo cutoff in the main. Other then that I haven't lost yet. I love this truck and I feel it is a advantage weight wise and handling on a indoor high grip track to run a 540 vs a 550 motor but once you go outdoors on a big track I feel that it changes.