Burk,
The largest tire you can fit under a '01 suburban without a lift is a 295/75/R16, anything above that your tires are going to rub, and you might have to do some minor fender trimmin wtih the 295's. Also, If you are going to go taller you also want to go wider. a 50 means that the tires height is only 50% of it's width. The general rule for getting larger tires is for every inch you go up you want to go out 2 inches. I have a bunch of mathematical solutions to figures out how much your speed, and gear ratios are effected. I'll post it as soon as I find it.