[DrKnow65]

I think at least for feigao's, from what I can gather, 35-40K is where they perform best. So for 4S (14.8v) a 6XL @ 2501KVx18.8V=37,014RPM or a 7XL @ 2145KVx14.8=31,746RPM.

If it's right that 35-40K is optimum for a feigao than the 6XL would be the best bet (for 4S), which means a smaller pinion for the same speed than any of the lower KV feigao's. I think this would load up less. Now the question turns to how much pinion does a 6XL need to get 40mph in your RC? (if if if if I am correct in in my understanding that feigao's are most efficient between 35-40K)

I have to imagine that the amps this setup is pulling could be the root of the problem... mabey 4S just isn't enough voltage for pushing the rig to 40mph without huge amps? Have you looked into modding the MM to run on 6S? I've got mine modded heavily (heat sink on both sides of the power(fet) board, pusher/puller 25mm fans in a "wind tunnel" setup, better capacitors on the power board, a zenier diode and 4 large capacitors on the power supply to the esc) I only run 3S but according to BrianG and GriffenRU these modds will make it run 6S without issue. Or just hold out another few years (sarcastic) and wait for the Monster Mamba Max to come out (early 08') it's made to run 6S...