if i get a tire that is 37x12.50 does that mean my rim has to be 12.50 wide?

 

No; a common rule of thumb is the tire shouldn't be more than 4 inches wider than the rim, but closer to 3 is a good thing to shoot for. Sidewall height is an important varaible, since the taller the sidewall, the better the tire can flex without deforming the tread block. More sidewall can handle a narrower or wider rim, less sidewall means you're more constrained. A 37" tire on a 17" rim has a profile height (one piece of sidewall) that is 10 inches tall, a 35" tire on 20s has a profile of 7.5", meaning the taller sidewall lets the tire flex outwards for a wider rim easier than a shorter sidewall would. Think of it as the extra 3 inches of rubber bends easier to a wider range.

Examples:
265/70R17 tires are ~31.5" tall and ~10.5" wide, and came stock on 7.5" wide wheels. 3" difference.
275/70R18 tires are ~33" tall, ~11" wide, and came on stock 8" wide wheels. 3" difference.
305/75/16 tires are ~35" tall, ~12" wide, and should be mounted on a rim ranging from 8-(9)-10 wide, with a 9" rim being the "optimal" size.

Most charts I've seen (primarily Nitto, since it's an easy reference) specs 12.5" wide tires on a 9 or 10 inch rim, ranging from as low as 8 to as high as 11. 17x10 or 16x10 are common rim sizes. I'd err towards the wider rim personally.

 

ok so i should be good with 8.50+. would about 9.5-10 be best??

and im confused about the whole sidewall thing sorry. is there a more simple way u can explain that?

 

What brand of tire? Usually the mfr's have a recommended rim width on their sites. For example, if you go to Toyo's site, their 37x13.50 M/T's have a recommended rim width between 8.5-11, with 11 being the optimum. I wouldn't use a 8.5" wide wheel on a 13.5" wide tire, but Toyo says it can be done!

The wider you can go, the better (within reason, you don't want to extend the wheel past the width of the tire, looks terrible).

 

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Got it now thanks