The kits I use and sell are limited to 17 amps, which allow for a top speed of 19-21mph on a 36v battery.
On 36v 15ah battery you can get 20 miles at 19mph..
Front motor kit.
I use it EVERY DAY for a 19mile at 19mph trip to work, where I charge it for 3 hours and I'm good to go for another 19miles