To answer your question, that's fine.Hi all, I'm looking at starting a budget conversion that is capable of 65+ mph. I found a possible donor car (BMW e30), and I can get a diff for it that is a 3.73:1 ratio. The car is an auto, so I can't really use the transmission. If I were to use a 10 or 11in forklift motor is it possible to get that kind of speed?
But more importantly, you're asking the wrong question.
It's not "is this motor able to rotate that fast" that is the challenge, "How much power can a motor provide?" is the challenge. So, technically yes, if you load down the motor down it won't rotate fast enough, but that's a power constraint, not a "is it safe to rotate this fast" constraint.
4000 rpm is perfectly within the rotation range of most electric motors. The bigger the motor actually, the lower the RPM has to be because the outer parts of the motor are under more centrifugal load and more likely to tear off.
What you're looking at is a 1200kg (2640lb) smaller car.
You'll probably need 15-17 or so horsepower to travel at 65mph (105km/h). That means a motor that can handle 12-15kW continuously. Plus some extra for acceleration and hill climbing (up to about double).
The cross-secitonal area of the car tells you roughly how much power it'll require to travel a given speed. The weight of the car tells you how much power it'll require to accelerate or climb hills, and how much energy you'll waste starting and stopping.
A 10 or 11" motor is plenty. A 9" is probably fine, power-wise.
http://www.electriccarpartscompany....-timing-1125-DE-Shaft-1125-CE-Shaft_p_68.html <-- Random link.
Even the smaller impulse 9", is good for 30hp continuously. Probably double what you need.