Where do tire and wheel widths come from?
 

I got a set of 17x9 wheels and 245/40R17 tires. 245 is supposed to be in mm, which is 10". Mounted on the wheels the tire is stretched. WTF is up with that?

I'm ranting because my wife just curb rashed one of my new wheels and while that is all on the fault of the driver if one were to mount a supposedly 10" wide tire on a 9" wheel one should have some freaking rubber to scrape on a curb before metal. I'm assuming the wheel width is the width of the barrel, is that correct? Based on what I see I could have saved weight and went with 17x8 rims for the same tire and had the outcome I was expecting.

Well that is a bunch of $hit. Using the tire rack's measurement method my supposedly 10" tires are only 9" wide. Why have numbers on tires if they can be off by 10%?!?!

I sound like a noob ranting about this, but after 11 cars this is the first one I changed wheels on and I'm not a fan of this system. Imagine if we used this much slop in all our measurements? How tall are you - I'm a median 6' which means I can be anywhere from 5'5" to 6'7". WTF?

Tire widths don't mean jack. I always look at the recommended wheel width guideline (min/max width) and base fitment on that.

-alex

there are a few tires with "rim guard"
like Michelin pilot super sport
slight help

That is actually what I figured I got, the tires are Hankook Ventus v12 Evo's and they have a pretty serious looking lip on them but it does very little when the wheel sticks out wider than the tire...

You have a BRZ and a Noble? When would you ever drive the BRZ? :)

Your tires are probably "squared" on that set up, especially a thick/broad shouldered sidewall tire (not to be confused with the "rim" guard". They may appear stretched if you have a thin/narrow shouldered sidewall tire.

Wheel widths are actually measured inside to inside bead. 99.9% of wheels have a standard .5" "J" flange tire bead on each side which then equals 1" total width outside to outside.

http://www.vintagewheelworks.com/images/offset.gif

OK, in that plot the rim width is what I would call the barrel and like in that plot my wheels have an extra 1/2" in lip, making them 10" wide. The lip sticks out at least 1/4" more than the '10 inch' tires.

Live and learn.

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Yeah then you probably have a thin/narrowed sidewalled tire. Plus you should have gotten 255s if you wanted more square. 25.4mm = 1" . 25.4 x 10 = 254 (so 255mm).

Oh crap, my memory failed me. I flipped the 5 and the 4 in the mm/inch conversion.

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