Quote Originally Posted by jaherbst View Post
It's a motorcycle, use motorcycle tires.
No it's not, technically. A motorcycle has two wheels and requires a rounded tire both fore and aft to corner properly. If you want some cheap entertainment watch the bar bikes (those with huge square tires in the rear) try to go around a corner sometime. And leave plenty of space between you in the event the rider is not good at rim-riding.

Because the Spyder doesn't lean while cornering it can use a flat car-type tire quite easily - especially on the rear. The stock Kenda rear tire is exactly that - flat (or square, if you prefer).

So long as the tire profile is the same as stock, the speed rating is sufficient for the use intended and the tire loading doesn't exceed its design specifications it should be fine to use a car-type tire on the rear of the Spyder.