Firstly, if you're afraid that you might put too much voltage into the motor, then why don't you just get a bigger motor. you could buy a 48V motor and then overvolt that.
Running a 36v motor at 60v (66% overvolted) should produce more power and a higher rpm than running a 48v motor at 60v (25% overvolted) as long as the windings can cope with the extra heat generated.
Secondly, If possible, you could increase the wheel size, larger circumference= more distance rolled per revolution.
By increasing the wheel size you also increase the gearing, making it harder for the motor to turn the wheel under load.
This would also mean less torque would be available at the wheel.
Ok, so I understand the first case, that was my bad, and I also understand the second case, but unless you're climbing lots of hills, or even one steep one, I didn't think that torque would be an issue. Is it? I mean would you really lose that much torque? If you would, then don't mind me, but I would like to find that out. would it be like you couldn't even move your bike from a stop?
I'll try an explain:
A hub motor will develop a limited amount of torque.(Torque is a force that turns, twist or rotates and is defined as force multiplied by distance.)
By increasing the wheel size you actually reduce the amount of force physically pushing the vehicle forwards.
A smaller wheel will give greater acceleration and be able to climb steeper hills than a larger wheel fitted with the same motor.
This is because the smaller wheel provides a greater forward force. (As it is effectively lower geared.)
However, if the wheel is too small it will run out of revs before it reaches the optimum speed where the wind resistance and frictional losses directly oppose the forward force of the bike.
If the wheel is too large, the maximum speed will be reduced because there is less forward force acting against the wind resistance and frictional losses.
Power and Gearing will both have an affect on maximum speed.
Power = Torque x Revs
Gearing = wheel size (With a gear less hub motor)
You're more likely to increase your maximum speed by increasing the power (supplying more volts/current) than increasing the gearing. (unless it is under-geared in the first place)
I hope some of the above makes sense to you.Alan