Tire Size Question

I currently have 235/75R15's on stock steel rims. My spare was a 215/75R15, but was not holding air so I took it in to Discount Tire (probably my first mistake) because someone told me they would fix it for free. DT told me that my spare was made in 1983 and that their policies did not allow them to repair anthing more than 10 years old. After telling them that I was currently running 235/75R15s on a 74 CJ5, they sold me a used Kumho Mohave AT 30/9.5R15 for $35.

My question is this - in the event I need to run my spare, what effect on the jeep, if any, could I expect from running the different tire size (30/9.5 for the spare vs. 28.8/9.25 for the other three)? Would this put my drive train at risk? etc?

Also, what is the largest size tire I should run on stock wheels? My 235/75R15's have plenty of life left in them, but it would be good to know for when I need to change them out.

 

The diameter should match. It won't hurt to run a different sized spare for a few miles, but it's not a good idea for any distance.

Go to tirerack.com and look at specific tires. The specifications will show the recommended rim size. The largest size BFG recommends for a 6" wheel is 235-75R15.

 

As Tim said, they should match. I you are running open diff's you would be fine using the odd sized tire as a temporary fix. If you have a locker, running an odd size tire will cause problems for you. It will put a lot of stress on the axleshafts. If you were out in the dirt it might be OK, but when you hit pavement you'll have two axleshafts trying to turn at one speed and two tires arguing about which speed they want to turn at.

 

Thanks Posimoto and Tim. Pretty sure I have open diffs. But since my jeep spends 1/2 the year 30 miles or more from the nearest tire shop, looks like I'll be going back to DT for the right size tire.