tubes the thing is you dont want a bigger sproket on the back, if your looking for more top end the rule of thumb is bigger on the front, smaller on the back. think about it like this, if you have 15 teeth on the front and 15 on the back of course with 1 revolution of the main driveshaft, the back wheel will accordingly spin one revolution, if you up the back to 30 teeth, 1 rotation of the front shaft will equate to a half spin on the back, therefore your motor has to work harder to get to the same speed as a 15 - 15 job. me i am running a 17 - 37 setup and she is nice and quick for a stock bike.
divy bro, what size sproket are you running on the front, for a single purchase your better off to change the front over the back as you get better results.