I bought some rims second hand today and just noticed that 2 are 15x7 and 2 are 15x8 I want to put 31x10.5's on these rims but I'm not sure if that will mess up the overall size of the tire thus resulting in badness..

any help?

 

No problems. I've run 285s and 33x12.5s combined over 80k on 7" with no probs. Fatter tire helps stay on the bead especially when aired down.

 

The tires on the 8 inch wheel will be a fraction shorter. Don't think it will be enough difference to pose a problem, mount them and use a soft tape to measure circumference that will tell you how much of a difference.

 

you'll be fine! dont even worry about it