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Old 11-26-2020, 10:37 PM   #1
bWRX
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Default Same tire on slightly wider wheels? How does that even work?

I know this is going to sound like a super dumb question to people who really know wheel fitment, but bear with me please because I am just not grasping this.

I'm shopping around for a second wheel set for my not-yet-in-my-driveway 2020 Levorg. OEM on this car is 225/45R18, rims are 18x7.5 with an offset (they call it inset here!) of 55.

The fenders are pretty tight on the Levorg and I don't want to inch up or down, but I was thinking...maybe go just a little wider? And sure enough, on a couple of the wheel shopping sites I have checked out, they are offering 18x7.5 or 18x8 options.

But the thing I can't wrap my brain around is that the tire size for both is the same. So I was looking at some of the tire manufacturer's pages and sure enough there is a pretty serious width range for tires, like up to two inches.

So how does that actually work? I don't work with tires so I can't understand how, if you put the tire on a wider rim, it doesn't flatten it out and reduce overall diameter. But all of the calculator sites I am looking at show that the rotational stuff is all the same.

Also is there any disadvantage to putting the same size tire on a slightly wider rim like this?

Thank you smarter people.
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Old 11-27-2020, 09:55 PM   #2
speedyHAM
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The sidewall of a tire can be though of in several ways. \_/ l_l /_\ The standard for most stock cars is the third option, when you fit a slightly wider wheel the overall diameter may get very slightly bigger, wider than that it will get slightly smaller, but the overall affect on tire outside diameter is very small. Keep in mind that a 12/32 tread depth tire changes by 1/2 inch diameter between when it's new and when it's down to the wear bars.
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Old 12-02-2020, 12:55 PM   #3
Mach V Dan
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Quote:
Originally Posted by bWRX View Post
...if you put the tire on a wider rim, it doesn't flatten it out and reduce overall diameter.
Near the center of the recommend wheel width, it won't make a difference. If you get the wheel wide enough, the sidewall will start to be at enough of an angle the tread will get closer to the wheel by a small amount. Let's assume the tire is a 245/40R18 on an 8.5" wheel, and you put it on a 9.5" wheel. The wheel would be 1" wider, so assuming the tire profile is made of straight lines (I know, bear with me) and ignoring any squishiness, the sidewall of the tire would move outward by 12.7mm. The sidewall height is 245*0.40 = 98mm. Pythagorean theorem says the remaining side of the right triangle is 97mm. So you've lost about 1mm of sidewall.

Dan
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Old 12-02-2020, 01:09 PM   #4
Samurai Jack
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Originally Posted by bWRX View Post

But the thing I can't wrap my brain around is that the tire size for both is the same. So I was looking at some of the tire manufacturer's pages and sure enough there is a pretty serious width range for tires, like up to two inches.

Also is there any disadvantage to putting the same size tire on a slightly wider rim like this?
If you are having clearance issues, why even consider going to a wider wheel?

As you have seen, tire manufacturers spec a range of wheel width for a specific size tire.
- If you noticed when you look at the tire specs, the sidewall, overall diameter, etc. are provided based on the wheel width.
- In the specs, they will say " measures wheel width ", which is typically in the middle of the wheel range spec'd.

The specs will slightly change from stated values if you go wider or narrower on the wheel, but the tire will still work within the wheel range provided.

Is there any advantage to a wider wheel on the same size tire? No, not really.
The attempt is to get a slightly wider patch of tire on the ground with a slightly wider wheel.

My guess is that no one has ever done a scientific study on exactly what gains / losses would result of a slightly wider wheel on the same size tire.
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