It probably has a revolution limiter to keep people fom over revving the engine floating the valves and blowing it up.
How many RPM's is it turning when it tops out? Ten mph in 1st gear that sucker is probably screaming anyway because 1st gear is always geared real low, high rpm's less speed delivers more pulling power/torque to the tires. 1st gear is to get you going, not for driving around.
