MOPED ARMY

if it's a TTLX you can do 2 things:

#1. cut the 2nd gear spring to make it shorter. That'll raise the shift point and have no effect on your top speed,

#2. Install a bigger front sproket. Your clutch will still shift at the same engine RPM, but since you have a high ratio it will be faster than 15 mph.

it all depends on what you want to do.