Tire Width vs. Wheel Width

[Inspector-Gadget]

This post was inspired by jps4jeep's recent post, found here:

http://www.jeepz.com/index.php?name=...6b5862da0196e9

I post what follows as a separate item because I didn't want to hijack or dilute that thread:

Background.

When I purchased the Jeep later to become infamous as the "Bubbacon," it came with Yokohama 33 x 12.5 Super Digger-Vs mounted on 11-inch wide rims. Those are the tires and rims you see on the Bubbacon in my sig-pic.

I also had at that time a set of 8-inch wide rims which I preferred only because of aesthetics.

So, never one to leave well enough alone, I trucked the mounted Yokos and the set of 8-inch rims to a Goodyear store with the intent to have the 33 x 12.5 Yokos swapped onto the 8-inch wide rims.

The Goodyear guys refused to install the 12.5 Yokos onto the 8-inch wide rims. Their explanation was that according to Goodyear tire-width to wheel-width fitting guidelines, the tires were too wide for the rims.

The Goodyear gurus went on to explain that 12.5-inch wide tires fitted onto too-narrow wheels would cause premature wear in the center of the tire because the center of the tire tread would bulge at normal road inflation pressures. Related to that were possible driveability and safety issues because of the overly narrow tire to pavement contact patch.

(One can imagine lawyers flying lazy circles over the Goodyear store at this point.)

Aftermath.

I thanked the Goodyear guys, took everything home, refinished the 11-inch wide rims and have since lived happily with the original tire-wheel setup.

By way of confirming the Goodyear experience, however, I did some web research on the tire width to rim width equation. I found the following table:



That table, rigidly interpreted, would appear to confirm my Goodyear experience.

Question. (You thought I would never get there)

Would someone who has "REAL" expertise, as opposed to my anecdotal expertise, please chime-in and expound at-length on this whole tire width to rim width question?

Given all the tire and wheel swapping being done by Jeepers, I think it would be a good idea if the tire-challenged, like me, were educated on the topic.

Thanks in advance to anyone willing and able to shed light on this problem.

(You know who you are.)

Regards,

Gadget

[PASmokeater]

I've heard ALOT about this topic on all of the boards.
A couple of things I have found out.

1. It IS possible to mount up to a 12.5 on a 8" rim, however as stated by you AND the Tire guys, I feel it is NOT a good idea. It can cause premature wear, and is also more likley to "pop the bead".

2. The back spacing on the rims are different, and you will get less rub with a wider tire in a wider rim. There will be no signifigant buldges on the sidewall, and will be able to have full turning radius with no rub.

When I was looking for tires and rims, I did tons of research on this subject, and the conclusion was always the same....when you go bigger, you need to go bigger. I went the 12.5 Yoko's, and put them on a 15x 10" rim. Best 10 $ decision I ever made.



Rob :P :)

[Inspector-Gadget]

Thanks, Rob,

I appreciate your taking the time to reply.

Not to mention the fact that since you agree with the tire gurus -- and me -- my opinion that you are a genius is confirmed!



Regards,

Gadget

[jhiggins]

Thanks for the info guys ...I have often wandered what size for what size ....LOL ...hehehehehehehe I just confused myself thanks...:)

[jeepinstien]

You can mount 12.5's on an 8" wheel.
I ran this combo for years and put over 100,000 miles on this combo without problems.
The benefit is more sidewall bulge and less wheel damage.
The only reason I'm running 10" wheels now is for the bigger footprint.
38's don't look right with a narrow footprint. I have seen 7" wheels with 12.5's but thats too narrow.
My .02 worth.

[clacasse]

The easiest way to get the correct contact patch is to lower the air pressure, but it is not a good idea to go below 28 psi if you do a lot of highway driving. I work part time in the Bj's Wholesale Club tire center (if anyone here has ever heard of it, it's like Costco or Sam's Club if you've ever heard of those) Anyway, what I usually do when someone want's tires mounted on too small a rim is I just explain to them any drawbacks that result from it (ones stated by PASmokeeater) and tell them what air pressure to maintain. Tire shops/centers are not held responsible for incorrect tire size due to the papers they have you sign when buying/mounting the tires. If you read the fine print it usually says that the customer is reponsible for correct tire size, air pressure, etc. So I don't see why they should've refused to mount them. 12.5s on 8 inch rims would fit just fine at the right air pressure but I would definitely recommend 10 inch.

[Inspector-Gadget]

Hello, jeepinstien,

Thanks for taking the time to reply.

My admittedly inexpert overview of this topic is that like other forms of engineering, the match up of wheel width to tire width is a trade-off of several tire performance factors.

To illustrate, a short quote from the Yoko Tire Co. web site:

"Choosing a wheel [width] near the middle of the range [considered acceptable for a given tire width] will give a balance between ride quality and handling. A wider wheel will improve handling at the expense of ride quality, while a narrower wheel will improve ride quality at the expense of handling."

That said, I don't doubt that many Jeepers are running wide tires on wheels considered too narrow by the tire manufactures. The reverse situation also surely occurs. But, in either case, it does not follow that doing so is a good idea.

So, the question here is NOT -- CAN -- one run a 12.5-inch wide tire on an 8-inch rim. Rather, the question is what are ALL of the factors involved in making an informed tire to wheel match up given the commonly available tire and wheel sizes?

Rob gave us an on-track good start in his post.

Yet, somebody with professional, honest-injun tire engineering expertise should put together a tutorial. One would think the tire manufactures would have already done that. This is not, after all, national defense security information.

EDIT: This might also make a good article for the "How To" section of a certain Jeep-centric web site.

With that information at hand, all in one place, then we who must compulsively tinker and mix and match would be better equipped to make choices and be more effective, satisfied consumers.

OK -- I had best stop now. I feel an Inspector-G, patented, full war paint, anti-tire manufacturer rant coming on.

Again -- thank you for posting, jeepinstien.

Regards,

Gadget

[erhild]

Not my experience, but I have heard that 12.5 inch tires on 8 inch rims will actually help keep you from breaking a bead when aired down...

I will be putting my 33x12.5s on 8 inch ARE Tailgunners in a couple of weeks... I'll be sure to post my real world findings...

[X_Factor]

I'm running 32x11.50s on a 15x7 rim although it does rub on tight turns. Personally, I believe that I should be using an 8 in. rim, but I just can't afford them right now. So, stock it is. But as soon as I can afford it (after the wedding), I'll go out and get some 8 in. rims. I'm not having any problems with uneven wear though. But then again, I check my air pressure about once a week and I'm very anal about rotating the tires.