I wouldn't go 48v - I read it from another board that it will fry your controller. Running it at 36v seems to be okay. I got my 2nd battery and as usual, I took it apart to tinker and made it 36 volt (temporarily). In this configuration, it seems like it has much more torque and speed has improved by another 5mph.

I've just order another battery/charger from www.monsterscooterparts.com and I will make it permanent to 36v. In the end, I will have a 36v main (in a bag) and a 24v backup.