Pretty sure a motorcycle will achieve the same speed when it has a full tank or one liter left. If you are referring to LPG powered scooters they have a pressure regulator so no change in speed until the very last dregs of the tank. Btw ChinaV suggested you balance it yourself. Most e-bike tyres are removed with a rusty screwdriver but a pair of tyre levers will cause less damage to the rim. It is entirely possible your hub/motor/wheel is out of round in which case there is not much you can do except replace it and no guarantee the replacement will be any better. Not trying to flame you mate it's just my kiwi sense of humour. Good luck with the problem and keep it shiny side up.