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Discussion Starter · #1 ·
Does anyone have experience with both Mickey Thompson ET drag R, sizes 315 60r15 AND 325 50r15? I’m currently running the 325 50r15 but desperately need new tires. So I’m looking at the 315’s. I can handle the height difference I’m pretty certain but I’m taxed out for reasonable width. So here’s the problem, 315 width is less than 325 width…unless you’re Mickey Thompson. I just got off the phone with their tech dept asking how or why the 325 would be measured narrower than the 315 or if we’re a misprint. They did not know, couldn’t explain it and were a little confused too. Even the rim they used for the 315 measurement was a smaller 9.5” rim than the rim used for the 325 measurements 10”.
Every tire size calculator calculates basically the correct sizes. Just not according to M/T. I could see there being discrepancies between different manufacturers or even different model tire of the same manufacturer based on the design. But there are both the same tire…different size.
Anyone know if the 315 60r15 actually measures .5” wider than the 325 50r15? Or better yet…why?

I have a couple pictures of the size difference. It’s from summits website but same on MT. The other is just a tire size calculator.
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I can't help with a fancy picture like that above but I can tell you the section width of a 325 tire versus a 315 tire is less than a half inch. Those numbers represent width in MM's.
10 mm is a little bit wider than 3/8th of an inch,which is definately less than 1/2 inch.
So a 325 is only10 mm wider than a 315 would be BUT the 50 versus 60 represent the aspect ratio and this is more difficult to express as its a percentage.
So a 60 series tire sidewall is 60 percent high as it is wide. While a 50 series tire is 50 percent high as it is wide.This is the measurement of where the rubber hits the pavement to where the tire attaches to the rim.
 

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Discussion Starter · #3 ·
Yes, agreed. The problem I have is that M/T has inaccurate, or at least seemingly inaccurate numbers and their technical department can’t explain it. So it makes it difficult measuring for a new tire size when the section width and tread width aren’t accurate. You are correct, they “should” be 10mm narrower than the 325 I’m running now. But according to tech, they aren’t and the best answer I could get was that it’s just how the American tire association (something like that) rates and categorizes the tire…so M/T literally told me that although it’s a 315, it’s wider than a 325. I know that’s wrong but it doesn’t help me measuring for a new tire size.
 

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I've found the same issue with Hoosier. The tire widths are typically correct but I've found the diameters to be off. The tires I run are supposed to be 29.3" tall but they always come in at 30". You just have to allow a little extra room.
 

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Your best bet is to check the manufacturer's website to see actual measurements. Just be aware of the rim width the tire was measured with as it can have an affect on section and tread width.

Very few tires are accurate to how the math calculates.
 

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Your best bet is to check the manufacturer's website to see actual measurements. Just be aware of the rim width the tire was measured with as it can have an affect on section and tread width.

Very few tires are accurate to how the math calculates.
You can't trust the numbers on the manufacturer's website. That's what I was saying. Both the Hoosier slick and QTP Dot tires that I run say they are 29.3" tall when they are really 30.0" tall. The circumference is supposed to be 92.0 and they come in at 94.25". The widths are accurate.

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The taller the tire or in this case also more sidewall height the bulbyer it will be thus overall / section width will go up to account for it. Though the 315 does have more tread width in this case.
 

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It's a misprint, tire sizes are an industry standard, but sometimes they may be rounded up or down a bit from actual.
 

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Good questions and I do not know. Helpful huh?

Their min. required wheel width seems awful tight/small; they listed 8.5
Ive seen 9.5" for 315..typically 10-11 recommended.
Id think the sidewall would be bulging bit time and perhaps lose some contact patch.

Do not know on the larger size. Let us know what you find out
 
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