I am currently working with the x-treme x600 scooter and the drive wheel consists of a fixed sprocket attached to the hub and other side of the drive wheel has a brake disc attached to the hub.
My back tire isn't exactly as precise as I would like it. I know that the chain tension changes as the wheel rotates so this probably means the hub that the sprocket attaches to is a bit off. (The chain goes from 0.25" give to 0.125" give as the wheel rotates).
Also the brake disc was a bit off side to side (disc is perfectly flat, so I'll blame the hub again) so it would cause the brake caliper to move slightly side to side (adding more resistance). For now I have the rear break disabled and I use the front break (it's stopping ok for me).
I've played with the idea of redoing the rear tire completely with a new rim and hub that would support a freewheel system so my electric motor isn't putting resistance on the scooter while coasting.
But I am curious how much of this lack of precision is acceptable in a scooter of this size?