MPH=(RPM X Wheel Diameter x Pi) ÷ (1056 x Total Reduction)
(6500 × 26 × Pi) ÷ (1056 × 18) = 27.9 mph.
So as you can see from the math your phone GPS was off or you were actually turning 8145 rpm or your reduction is actually 14.36/1. The only way you the GPS would be correct for 35 mph with the data you provided is if this was a down hill speed. These 4 stroke 30ish cc engines only have around 1.5 hp. With that low of an hp about all you'll squeeze out of them at level ground speed is 29 mph.