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Old 08-13-2004, 01:53 PM   #1
sonicblue
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Default Upsizing: Matching wheel width to tire width and adjusting offset

I'm confused on something.........

My OEM specs for my TL are:
* tires: 205/65-15
* wheels: 15x7

I'm considering upsizing so I was browsing Tire Rack for wheels. As a check, if I selected 15x7 as my size, it did, in fact, show me a recommended tire size of 205/65-15. Here's what I got (selected wheel -- recommended tire):

* 17x7 -- 215/50-17
* 17x8 -- 215/50-17
* 18x7.5 -- 215/45-18

What I find so confusing is that each wheel is a different width but the recommended tire for each is 215mm wide. NOTE: I DO understand that the recommended tire size ON THE WHOLE is the best fit in terms of maintaining the same overall circumference as the OEM wheel/tire combo. That's not the question. The question is, how can the same width tire fit a 7, 7.5 and 8" wheel all at the same time? One of these *must* be a better fit than the others, right?

For instance, the stock 205 tire is almost exactly 8" wide, while the rim is 7" wide - a one inch difference. A 215 tire is almost exactly 8.5" wide, so does that mean it BEST fits a 7.5" wheel (maintaining the same one inch difference)?

While I'm at it, I wanted to throw in this question: is there an easy way to reflect offset change when upsizing? For instance, I believe my OEM wheel is +42. Say I go with a 10mm wider tire but keep the same offset - the additional width is split evenly, the new tire is 5mm closer to the strut and 5mm closer to the fender. If I didn't want to tire to protrude toward the fender any more than OEM, than I would go +47 offset. If I didn't want the tire to be any closer to the strut than OEM, I'd have to go +37. Do I have that thought process right? If so, what should really be my goal for how the new bigger tire is positioned? Is this only something that an manufacturer-specific tech could help with?


I did read Luke's stickies and links to the Rack and didn't see anything - apologies if it's been answered. Please direct me there if so.
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Last edited by sonicblue; 08-13-2004 at 02:03 PM.
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Old 08-13-2004, 02:12 PM   #2
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Most tires have a range of rim sizes they will fit. If you look at the detailed specs for any given tire, it will tell you what the "acceptable" size is and also what the "measured" size is. The measured size is the preferred wheel size they use to measure RPMs, section width, etc.

I didn't look for your tires, but for a 215/45/17, the measured width is 7", but the tire will fit up to an 8" rim no problem.

EDIT: I just looked at a couple tires, and they both listed the approved width as 6" - 7.5", with the measured width at 7" for a 215/50/17. Depending on the tire, 8" may be too wide.
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Old 08-13-2004, 03:32 PM   #3
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This seems like an appropriate place to ask this question, rather than making a new thread. Right now I have stock wheels/tires on my WRX. I'm hoping to upgrade to 17" OZ Supperleggaras soon, and tirerack recommends a 225/45-17 tire with those. Would that keep the same overall diameter as my stock tire/wheels?
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Old 08-13-2004, 03:48 PM   #4
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Use the following terms: 225 / 45 - 17 ==> S / A - D, where

S = section
A = aspect ratio, expressed as a percentage (0.45, for instance)
D = diameter

To calculate the total outside diameter of the wheels, do:
[ S / 25.4 * A * 2 + D ]

Using the above example, you would get:
[ 225 / 25.4 * 0.45 * 2 + 17 ] = 24.97"

Stock tires for the WRX are 205/55-16, so that would equal:
[ 205 / 25.4 * 0.55 * 2 + 16 ] = 24.88"

That's a difference of less than 0.5%, so - yeah - pretty durn close!
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Old 08-13-2004, 04:21 PM   #5
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Hey sonicblue, you want to know what size fits the BEST.

Fliz gives a range of acceptable sizes.

Porque tells us what he's getting.

And I don't have an answer either, but definitely curious on how it all works too.

Where's TireRack with the answer?
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Old 08-13-2004, 06:30 PM   #6
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the width of the tire is the width of the contact patch.

a 7" wheel can fit a wide range of widths.

however, mounting a thinner tire on the wheel will result in a stretched look to the tire, whereas mounting a very wide tire will result in a bloated look and often not recommended due to inconsistant contact patch and uneven tire wear as a result of the bulging contact patch. (tire fitment for racers vary but I'm talking about general use here).

tirerack probably reccommends those sizes because of fender and/or strut clearance issues. with a tire that's wider than 215.

just for reference, a 7.5" wide wheel and a 215 tire will ahve mostly a flat sidewall profile in relation to the wheel. The STi has 225's on 7.5 wheels and it's slightly bloated. if 215s are the only tires that will fit under your fenders, then I would stick with a 7 or 7.5" wheel. you're less likely to curb the wheels with a 7" and 215's though.

also, as long as the overall tire circumference is around +/-5% of the stock circ, you'll be fine. if anything I'd lean towards the slightly larger one than the slightly smaller one since your speedo is most likely optimistic like all speedo's made these days.
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Old 08-13-2004, 08:59 PM   #7
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That's good info, az, but the question is still unanswered. Tire Rack has recommended the same tire for three different wheel widths. I know you can physically mount a certain size tire on different wheels with differing resulting appearances and potentially detrimental effects. I also realize that my fender well may only fit a certain size comfortably.

But imagine we're past all that. Here are the givens:

* I know my exact OEM tire/wheel specs, including offset
* I want to upgrade and figure out that I can safely fit a 215 tire.
* I know I want to put 17" rims on the car.

Answers I can figure out:
* For a 215 tire and 17" rim, the best aspect ratio to stay as close to OEM circumference as possible is 50.

What I do not know:
* What width wheel should a 215 ideally be mounted on?
* Does that answer depend on the wheel diameter, or is it simply width-width matchup?
* Also, this has been getting ignored, can I figure out what the ideal new offset should be?

While answers like "you won't curb your wheel if your sidewall sticks out" and "if your offset is too low, you'll rub your fender" are practical implications, it seems to me there is a "right" answer that is more based on what is best for tire life/handling (wheel width) and best for your bearings (offset).

Thx to all for good info so far, but I hope this clarifies what I'm actually looking for.

Last edited by sonicblue; 08-16-2004 at 08:51 AM.
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Old 08-14-2004, 09:57 PM   #8
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4 character bump
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Old 08-14-2004, 10:36 PM   #9
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sonicblue, as you know, each tire has a recommended range of rim widths. IMO, going with something towards the middle of the range will be a better fit than going with either extreme. It will put less stress on the tire that way.

As far as the offset issue, in this case I'd try to stick as close to the factory offset as possible if I were you. That should help prevent any problems down the road.
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Old 08-14-2004, 11:36 PM   #10
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oops, just posted Subaru specific info and just realized you have an Acura.
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Old 08-15-2004, 10:29 AM   #11
sonicblue
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yeah, MO, I realize this is a Subaru board but it seems there are a ton of questions still about these issues. I thought if we could get the general knowledge laid out then anyone could figure out what they need from there.

Still waiting for Luke to drop some info......
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Old 08-15-2004, 10:59 AM   #12
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Quote:
Originally Posted by aznatama
the width of the tire is the width of the contact patch.

a 7" wheel can fit a wide range of widths.

however, mounting a thinner tire on the wheel will result in a stretched look to the tire, whereas mounting a very wide tire will result in a bloated look and often not recommended due to inconsistant contact patch and uneven tire wear as a result of the bulging contact patch. (tire fitment for racers vary but I'm talking about general use here).

tirerack probably reccommends those sizes because of fender and/or strut clearance issues. with a tire that's wider than 215.

just for reference, a 7.5" wide wheel and a 215 tire will ahve mostly a flat sidewall profile in relation to the wheel. The STi has 225's on 7.5 wheels and it's slightly bloated. if 215s are the only tires that will fit under your fenders, then I would stick with a 7 or 7.5" wheel. you're less likely to curb the wheels with a 7" and 215's though.

also, as long as the overall tire circumference is around +/-5% of the stock circ, you'll be fine. if anything I'd lean towards the slightly larger one than the slightly smaller one since your speedo is most likely optimistic like all speedo's made these days.
What I'm taking from this is that I should I go with a 215/45-17 tire for the 7" wide OZ Supperleggara, rather than the 225/45-17 tire tirerack.com recommends. Is this correct?
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Old 08-15-2004, 12:55 PM   #13
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here is a good site too compare tire sizes

http://www.miata.net/garage/tirecalc.html

hope it helps
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Old 08-16-2004, 08:49 AM   #14
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No, it doesn't -->
Quote:
Originally Posted by sonicblue
Answers I can figure out:
* For a 215 tire and 17" rim, the best aspect ratio to stay as close to OEM circumference as possible is 50.
Please everyone, read the whole thread before answering. Let me summarize again:

1) Given that TireRack is showing me that the same width tire is recommended for THREE DIFFERENT wheel widths, how do I determine what is the BEST wheel width to actually use?

2) Once you go with a wider tire, is there a way to figure out how to adjust your stock offset for that additional width? Obviously, what fits is a consideration but I understand the real issue to be, what's best for your bearings. So, how do you figure what's best once you change stock parameters?
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Old 08-16-2004, 02:28 PM   #15
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Quote:
Originally Posted by sonicblue
1) Given that TireRack is showing me that the same width tire is recommended for THREE DIFFERENT wheel widths, how do I determine what is the BEST wheel width to actually use??
The best wheel width for dry handling performance is the widest. For every 1/2" increase in wheel width you gain .2" in section width, which will translate to a wider contact patch and better dry handling. This ratio of wheel width to section width is also mentioned on Tire Rack in their technical tips. This is a critical issue on the track, but on the street it's only going to matter if you drive agressively.

The range of wheels sizes for a given tire size is based on safety, not performance. The manufacturer has tested the tire size in question on those wheel widths and has found sufficient support for the tire at those widths. They choose one wheel width as a reference for a specific tire size to provide comparisons with other tires in their lineup on section width, tread width, etc. but there is no reason why you have to use that reference setting. It is not the "recommended" setting, simply a reference point for comparison.

For example, you can safely run a 215/45x17 tire on a 17x7, 17x7.5 or 17x8 wheel. The 215/45x17 on the 17x8 wheel will give you the widest section width and the best dry handling - assuming the combination doesn't rub. I'm not recommending that particular combination - a 235/40x17 would be a much better choice in tire size with that wheel width from a performance standpoint - but it would work fine and would maximize the dry performance potential of the 215/45x17 tire.

Quote:
Originally Posted by sonicblue
2) Once you go with a wider tire, is there a way to figure out how to adjust your stock offset for that additional width? Obviously, what fits is a consideration but I understand the real issue to be, what's best for your bearings. So, how do you figure what's best once you change stock parameters?
The lower the offset, the wider the track and therefore the better the cornering performance. Of course, that also puts your tires closer to the fender so you may rub. Many on this board have had their fenders rolled to allow them to run the widest possible tires on the widest possible wheels - which usually translates to lower offsets - for performance gains in dry handling. This approach may sacrifice wheel bearing life, however.

The factory offset will provide the best protection for your wheel bearings, so the closer you are to the factory settings the better you are from that standpoint.

Going with a higher offset will move the tire closer to your struts and so your rubbing potential is now there instead of at the fender. On Subaru's, this usually only becomes an issue with aftermarket coil-overs.

The widest wheel with the offest closest to stock is usually the best overall choice, assuming that it won't cause rubbing at the strut. This provides a balance of a wider track and reasonable protection for the bearings.

The "correct" offset for Subie's ranges from 45 to 55mm. I would assume that there is a similar range for the Acura, but I have no idea what it might be.
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