And here is the calculation with the 13 tooth sprocket:
If you want the bike to have more than a 21 MPH top speed then the rear wheel sprocket will need to be replaced with something smaller, but if you are okay with 21 MPH then installing a 13 tooth sprocket on the motor will do the trick.
Our gear ratio and miles per hour calculator is located on this page: http://www.electricscooterparts.com/motorwheelgearratio.html
I don't want to get it wrong I need a new #25 chain but what size
I do not know exactly what size #25 chain is needed for the sprocket modifications that you are planning to make. I do know that you will need the heavy duty #25 chain though. We have a chain length calculator which can help determine the chain size needed on the following page: http://www.electricscooterparts.com/chainlengthcalculator.html
It can sometimes be difficult to take an accurate center measurement on the axle though. To play it safe you may want to consider buying a little more chain than you need and breaking it down to the right size after you receive it. That way you can be sure that you do not buy too small of a chain for the project.
Chain can be broken with a chain breaker tool such as the one that we sell on our tools page - or with a lot more work by using a hammer, punch, and bench vise.
Once you do figure out what chain size is needed it would be really cool if you could post the number of links here so other people doing the same modification will know how many links of chain to order.
Chain Breaker Tool