I had my dealer check the speed on a 2012 Super Z HyperDrive with the 24" tires, and a 2013 Super Z HD with the 26" tires by GPS. The 2012 Super Z with 24" tires after averaging four runs was at 15.7 mph. The 2013 Super Z with 26" tires after averaging four runs was at 17.2 mph. This may have been done before but I have never seen this information in print. Now, for all those such as myself with wondering minds, we have accurate speed numbers based on GPS.