Interesting... Never had a trailer. So, while I knew they made different tires for trailers, I never gave much thought to why. There are reasons not to swap either way. Just did a little quick google read... While most people know the trailer tire is not speed rated for a car (but seam to forget that when pulling a trailer) They also don't have as good of tread for both wear and traction. Going the other direction- car tires don't have the heavy sidewall that trailer tires do. This is to reduce sway.