I've had the same experience when I use a GPS on my Dixie. Although it's top speed is just over 15 MPH, the working speed is somewhat slower. When I cut, I listen to the blades and watch the results in the grass. If I hear the blades slowing down then I slow the mower down. If I see the blades missing grass or weeds then I slow down. On some lawns I can rip but other lawns make a 15 MPH top speed just downright useless.