Look on the drivers side door, see what tires came on your truck. More than likely, it came with a 235/75/R15s on it. Your tires are 31 inches tall , 11.5 inches wide, a 31/10.50/R15, would be a inch narrower, in between the 235s, which are probably 29 inches tall, and 8-9 inches wide- Hope this helps!
If ya want to go bigger, the 33/12.50/R15s would be the next step up!
