I meant to say that getting the correct gearing is not a factor. I have a 32 and could easily change the front to achieve a higher gear ratio.

The problem occurs when the motor does not have enough power to overcome the drag and most of it at higher speed is aerodynamic. Small improvements go a long way

Bicycle Speed (Velocity) And Power Calculator
Bike Calculator
Here some data for bicycle power requirements for pedaling:

typical fat tire back used for many motorized bikes with fat smooth tires, 157 pound rider, with 160 watts will achieve 14.5mph

same rider, racing bike, same conditions with 160 watts achieves 19.1 mph

same rider, triathlon bike, 160 watts achieves 20.2 mph

These are VERY SLOW speeds. As you go faster the power requirements sky rocket.

Check out the websites listed and play with the numbers and you will see what I am getting at.

Also, try entering a 500 pound rider compared to the 150 pound rider using 1000 watts riding on the hoods. The speed of the 500 pound rider slows down by about 1.5 mph (given same drag)