cycleguy04

Moderator, please sticky! This is what many people want to know about wheel fitment, but it has never been described in this way!

Wheel Fitment Explained

With wheel fitment for a third gen with stock axle, to get the outer surface of the tire flush with the very outer point of the fender use what I call constant 'A'. We've all heard of backspacing and get caught up on what the correct backspacing will get your tire even with the fender lip. That is not the correct way of thinking.

Constant 'A' is 4.25" on a third gen with third gen axles and can be defined as the amount of wheel outboard of the mating surface of the wheel. Subtract the constant 'A' from your desired wheel width and that gives you your needed backspacing.

For example:

10" wide wheel - A = 5.75" backspacing.

8" wide wheel - A = 3.75" backspacing.

This equation takes into account tire bulge. About an inch. If you are interested in how to determine tire bulge read on.

Determining Tire Bulge

Tirerack.com has specs on their website that allow you to figure out tire bulge. Section width - wheel width / 2 will give you tire bulge.

For example:

A 255/60R15 tire's section width of 10.2" - measured (suggested) rim with of 7.5" / 2 = 1.35" bulge.

Last Words

If you think you may have a larger or smaller amount of tire bulge the wheel fitment equation can be adjusted. Post your wheel tire combination and I will be glad to help you figure it out.

I will post pictures soon of a good example. 255/60R15 tires and 15"x8" wheels with 3.75" backspacing.

In the mean time:

paul_huryk

Your math is off a bit:

Wheels from lip to lip on the barrel run about 1" wider than the stated wheel width - so a 10" wheel is actually about 11" from edge to edge.

Tires are never the same size in terms of section width when you compare the same tires (275-40-17 as an example). BFG always run 10 to 15mm wider than spec and others are dead on. Do not assume they are all exactly the same.

Also, "front spacing" as you describe it is more like 5.5", not 4.25" from the wheel mounting pad to the fender lip.

cycleguy04

Everything you said probably is true, but your last comment. Remember my calculations allow for about an inch of tire bulge, so as a general rule, the equation should work for finding the right WHEELS. I know I'm on the right track. Help me modify this a little bit so the general enthusiast doesn't to become a wheel and tire expert to get a nice looking set. Crunch a couple numbers, order, done.

This worked for my setup. I would really like to put it to the test on some different wheel widths and tire sizes, but I can't afford to do that. I will have to start compiling data and posting it here to either debunk or prove this theory.

jhev1

So if I bought an 8" wheel with a backspacing of 4.5, I would be off .75. Does that mean that the wheel would be recessed an additional .75 or protruding .75? As an inexperience person with all this, I never knew there was so much involved in finding the right wheels!! There are a ton of things to consider!!

PATRIOTIC1911

iTrader: (2)

The higher your back spacing, the more inward your wheel will sit from the fender.

jhev1

Thanks. Would a .75 difference discourage you from purchasing that particular wheel? Is it enough to cause any problems?

PATRIOTIC1911

iTrader: (2)

Don't think it will cause any problems, though I'm not positive. I wouldn't buy the wheels personally because I would prefer more of a flush look with the fender line.

87350IROC

iTrader: (2)

No, Paul is correct. You didn't invent the term front spacing, it already exists.

Front Spacing roughly equals wheel advertised width + 1 - back space.

FS = wheel_width + 1 - BS

There really is no mystery to wheel sizing. A straight edge and a ruler is all you need. I know from experience on this forum that 95% of people refuse to do their own measurements because either

1. too lazy

or

2. Don't trust themselves and would rather trust some dude on the internet

cycleguy04

Yeah, your tires would be .75 inside the fender. Some people actually prefer to have the wheels in a little more. With the tires even with the fender you run into the situation of the tires possibly hitting the fender lip and damaging the tires. You can get around that by 1) stiffer leaf springs, 2) airbags to suppliment your leaf springs, or 3) rolling the fender lip to get it out of the way.

If you take it easy though, you shouldn't have a problem. Depending on the tire diameter, it would take a pretty hard bump to push the axle up enough to hit the fender lip. In cornering, the side that compresses will actually roll a little away from the fender lip, so that isn't a worry. Just a really hard hit to the rear end.

So you have two choices. Get the wheel and tire combination you mentioned without any worry of the tires rubbing or get one that is flush with the fender and either be mindful of over-compressing the rear axle or possibly have to do some sort of modification to prevent them from rubbing. Your choice. I prefer the flush look.

cycleguy04

What's up Everette? Eff Dub!

Alright. Remember I'm trying to simplify this to make it easier for the average enthusiast. Maybe the correct way to go about this is change the term front spacing so I don't confuse the people who actually know what that is and just leave the +1 OUT of the equation. All your doing is adding one to the constant, which I have already done.

The point of this thread is so that somebody with a third gen with a stock axle can come one here, use my equation, and order up some wheels with less headache.

Fact is: it works.

paul_huryk

The equation is sound - the math does work in general. But your calculation of 4.25" of front spacing is way wrong - I went to my cars and measured both the other day and it is in fact 5.5" from the mounting hub to the edge of the fenderwell, others can verify. If you plug in 5.5", your equation works on the wheel side for the most part. Tire bulge is dependent on 2 factors, so you can't just calculate tat - it must be measured.

With these cars you still need to be precise on wheel measurements. If you are off by 0.5" on wheel backspace (say 5.75" vs. 5.25") on a 17" x 9.5" wheel, they will not fit in the front.

That said, there is no easy way to measure yourself if the premise is incorrect in terms of how to calculate. I think that is why there are so many threads on wheel fitment here as 1) people don't know how to research TGO or the web properly 2) seems like newbies for the most part would rather start from scratch than take others advice from their experiences.

86iroct5

http://www.rimsntires.com/specspro.jsp
this should be bookmarked on everyones pages. i use it all the time

PATRIOTIC1911

iTrader: (2)

Yep. This is exactly where I go also.....great site.

cycleguy04

Went to the site. I see how that could be useful. If somebody uses the math described in post 1, they will not need to measure anything as long as they have a third gen with stock axles.

Also, I have edited the post to omit the use of the term front spacing to avoid some confusion.