[bdebde]

It is not so much efficiency numbers, it is more about current (amps). It takes a given amount of watts to move a vehicle a given speed (varies with every set up). Say it takes 1000 watts to move the car, you could use 10 volts which would draw 100 amps and things would get hot, or you could use 100 volts and only draw 10 amps and everything runs cool. More heat in the components can cause efficiency to go down as well as more power wasted to heat.

Motor efficiency doesn't change much no matter what the kv but y winds are often more efficient (lower kv than the d counterpart). My lehner 1930/8 is rated at 90% efficiency as d wind but when reconfigured to y wind it is rated at 92% for example.