Rim Sizes

Ok so I admit it, this is the first time I am buying rims for any vehicle. I'm putting 31x10.5's on my '98 XJ, of course on a 15 inch rim. I get that part, but what about the other number? Basically what should I be getting for that size tire, 15x8? 15x10? Any help for this clueless soul would be greatly appreciated.
Thanks
Ryan

I run my 31's on the stock 15"x7" rims. I think a lot of people that upgrade go to 15x8's. The other thing you want to look at is backspacing, but someone else will have to explain that one...

I have 15x8s on my 98 XJ sport 8" lift 35" Xterrains. I went with the smaller 8" rim for one reason, so the tire would bulge out around the rim thus limiting the amount of damage that could be done to the rim.

The rim is sucked back in compared to the tire side walls, so when I scrape the sides of the tire it's still not even close the rim. Something to keep in mind if you want totake care of your rims. I drive mine on the street as well, so I do care what they look like.

Plus when you air down your tires it keeps a better bead on them.

My .02 cents

I was thinking that it would make sense to go with a rim that is narrower than your tires for just that reason, I wasn't even sure that the second number was the rim width, though, so this is good it means I can get a cheaper rim than i expected thanks

I run 31x10.50s on stock 15x7 rims. The inside of the tires rubs against the lower control arms when the steering wheel is turned all the way to the stops. You can shim the steering stops to avoid this, live with it, or buy rims with less backspacing to push the tires farther outboard. But if you go with reduced backspacing, then the tires may hit the flares when the suspension compresses.

How is backspacing determined, is it a number given on the rim, and what is the stock rim's backspacing? if anyone knows, that is...

Stock 15x7 will work just fine. 15x8 is just about ideal. 15x10 will work, but is getting a little bit wide.

My stock rims are rusty/spraypainted, messy, junk, etc. So I'm gettin some half decent ones(hopefully)

15X8 with 4 1/2" to 4 3/4" Backspacing is the most common recommendation for a 31X10.50 tire.

Your tire manufacturer should have some specs with a minimum and maximum width for a given tire size. As an example, BFG specifies 7.0" to 9.0" wheel width for their 10.50 tires and 8.5" to 11.0" wheel width for their 12.50 size. The guys at my tire store cried like babies when I asked them to mount my 12.50 section width tires on an 8" wheel. The majority of off-roaders will recommend a wheel that is narrower than the minimum recommended by the manufacturer so that the tire sidewall will protect the wheel and also feel that narrower wheels help keep the tire bead seated at low inflation pressures. I'm not so sure I'd want to sacrifice a $180.00 tire to protect a $50.00 wheel.

To complicate things, when you go to the tire store and ask for a 4 1/2" backspace wheel, the guy behind the counter is as likely as not going to return a blank stare and ask about offset instead. To convert from Backspacing to offset you can use the following formula:

Backspace - (width + 1)/2 = offset
4.75" - (8" + 1")/2 = .25" positive offset (SAE)

To convert the answer to metric, multiple the answer by 25.4. The example 4 1/2" and 4 3/4" offsets on an 8" wheel would have metric offsets of 0mm and 6mm.

P.S. I hope you're planning on a 3" lift to clear these tires.

MaXJohnson said:

Backspace - (width + 1)/2 = offset
4.75" - (8" + 1")/2 = .25" positive offset (SAE)

Click to expand...

So if I derive a physical meaning from your formula correctly, is offset the distance from the centerline of the wheel to the mounting face? I understand that backspacing is the distance from the inside of the wheel to the mounting face, but had always wondered what offset referred to.

Lawn Cher' said:

So if I derive a physical meaning from your formula correctly, is offset the distance from the centerline of the wheel to the mounting face? I understand that backspacing is the distance from the inside of the wheel to the mounting face, but had always wondered what offset referred to.

Click to expand...

Correct.

Excellent diagram... we should archive that somewhere.

I was going to say just turn your rim upside down and put a ruler across it then measure with a tape measure from the surface that mounts to the drum to the bottom of ruler. then you have your back spacing.

Just like the diagram!

98XJsport, As a fellow new guy to the world of XJ's, you are probably just as confused with those diagrams as I am. I run a 15x7 rim with 4 inches of backspacing, and I have no rubbing. I have a 3 inch lift, and 31x10.5 BFG All terrains. I would not recommend the stock backspacing of 5.25 inches, as I run that for a year and had nothing but trouble with rubbing. Also, a 3.75 inch BS will work too.

Thanks everyone for your help, and yes I plan on a 3" lift, cramming 31s on stock would be a bit to much fender trimming lol.

I have a 98 Sport and I tried to run 31x10.50x15 on a 15x7 rim w/3.75 backspacing. I had a horrible time with the tires hitting the flares. I ended up doiong some major trimming in the rear and minor trimming in the front to make it drivable until I put on my lift.

Later,
Rough