I put a 21 x 3.5 RC Components Nitro wheel on the front a few weeks ago, added the chrome forks, and lowered the bike. Now I can't get around the stock wheel and tire on the back.

I found a retailer online that has a 16 x 5.5, for much cheaper than retail, and I'm wondering if it will fit on the rear. I know the stock wheel is 16 x 5, but how much wider can you go on a wheel? And, if this wheel will work, what size tire should I go with; a 50 or a 55?

Any help you can give will be appreciated!