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tire measurement diffs?

 
Old 09-11-2006, 05:33 PM
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Default tire measurement diffs?

Can someone explain the difference in tire sizing between the metric (ex: 255/75R17) and the SAE/American/whatever (ex: 33X9.5QR15) to me? When speaking of putting 33s on, does that translate to a metric-type tire, or does it have to be the other measurement (like 33"x whatever)? Thanks.
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Old 09-11-2006, 08:41 PM
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Okay, when you see 255/75R17, the 255 is the measured width of the tire tread in millimeters. The 75 is the ratio or percent of sidewalls in relationship to the tread width. In other words, the height of the side walls is 75% the width of 255mm. The R stands for radial construction and 17 is the diameter of the wheel it's mounted to.

In its practical sense, a 255/75/17 tire really equals = 32.1x10/17

Typically, most tires made for a 16" wheel or larger will come in metric. Does that make sense?
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Old 09-12-2006, 08:28 AM
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jeez...little did I know that I'd be learning higher math on a jeep forum.:rolleyes: Thanks, though. I did know that about the metric, I just wondered why people don't say, "...and I'm gonna stuff some 295/85s in the wells", instead of saying 35s or whatever, y'know?
I guess what I was trying to ask was why with Jeeps and 4x4s in general, people talk about the American dimensions of the tires, and if there is any difference, ultimately in what I order (probably gonna get BFG all terrain T/As).
For example, I have a 350z, and recently got some new treads in the back...they are 18" wheels, but I never saw anything referenced in terms of inches as far as the actual tire went. They are 245/40 18s. Not to belabor the point, I just wanna make sure I'm getting the right thing.

Last edited by luna95; 09-12-2006 at 08:31 AM.
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Old 09-12-2006, 08:59 AM
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:o LOL!! I apologize for the unwarranted tire lesson.

As far as why Jeep/Off-Road vehicle guys refer to tires in inches go.... I have no idea other than the fact that it's easier to get an idea of what were talking about.... comparing apples to apples kind of thing. As you know, common off road tire sizes in the standard 15" wheel variety have always been 31's, 32's, 33's, 35's etc. and so when you say something like "I got me a set of 285/75R17", it's harder to get a feell for how that would compare with what most of us have on our Jeeps now. That is of course, unless you knew what it's conversion is
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Old 09-12-2006, 07:12 PM
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Originally Posted by wayoflife View Post

In its practical sense, a 255/75/17 tire really equals = 32.1x10/17
shoot, that's all you, brotha. look at you converting metrics to decimals. that's why i dropped calc III.:confused:

really, though, thanks. much better to get more info than less.

btw, after looking at the pix from your test drive, now i KNOW i'm doing the right thing by putting some 33s on.
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Old 09-16-2006, 10:01 PM
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Why worry about math when we have the WEB for a resource. Check out Procomp Tire's site and you'll find a tire size conversion calculator. Just go to www.procomptire.com and click on any of the tire images. Then at the top of the pave you'll see header boxes, the far right light-blue box says converersion chart. Click on this and can check any P-metric tire size and get a standard size conversion to compare. I'm sure you can search out other conversion charts on the web.

Remember that almost every tire maker rounds up from the decimal point, so a 34.5 dia tire will be sold as a 35.
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Old 09-22-2006, 07:52 AM
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You beat me to the punch jeepminded. I was going to say that the inches height system is similar to the magic they use sizing crt monitors. It seems like tires fall shorter than a half inch in most cases, when compared to the claimed size. More like if it's an 1/8 bigger than 32, then it's a 33.
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Old 09-22-2006, 10:59 AM
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With cars, there are a lot of variables you need to fit. With 4x4 tires, it's almost always been about HEIGHT. Back in the day, you only had 15" wheels (or 16.5" if you drove a 1 ton truck) And all the tires came sized:

DIAMETER x WIDTH - RIM

We soon got to put the 'R' in for radial, but many of the best tires of the day were bias ply. And let me tell ya, the above sizes were a lot better than seeing a size of:

Q78-16

Which is one of the more popular 35-36" Super Swamper Tires, but was a popular size method for large tires up to the early 80's.

Or even

600-16

Which of course is the tire size of the original Willys CJ2a.

Now, with the metric sizing, you get a lot of information, but you have to piece together several items to get an idea of just how tall we are talking. And even then it is just an estimate as the actual sizes can vary greatly. No matter what you want, it's best to go find the manufacturer's true size chart for more accurate dimentions.
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