Well...... I am answering my own question.... after much research, if you have a late model Yamaha electric, ( short of changing out the on board computer or motor ) , for a speed increase you have to purchase the speed "code" through a Yamaha dealer. This requires the cart have headlights, tail lights and a horn, soooooo, there you go. The Yamaha dealer then enters the speed code into the controller for a 19/20 mph cart instead of the factory set 14/15 mph cart. Thats my story and I am sticking to it !!
Hope that this helps someone with a Yamaha speed increase question ....
Cheers, Johnp