40k is a speed motor, not as fast as 45k but pretty quick
(think 30k is a bit sensible snap)
keep it on 7.4v - it might run a bit warm
but don't ffs try anything higher than 7.4v or it will wreck
20k-25k you can run on 11.1v most likely
but the faster motors - yeah keep them on 7.4v
it will get a bit warmer than stock motor as it's running faster of course
will drain your battery quicker to as it draws more amps etc....
it's ok but keep the voltage sensible @ 7.4v max