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Old 04-19-2016, 09:17 AM
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Join Date: Sep 2015
Location: UK
Posts: 210
Confused about stated tyre sizes?

Am I missing something?

Tyres are measured: diameter x width x rim size, yes?

My tyres say 31 X 10.5 x r15 but measure 29.5 x 10.5 x 16

And my spare says p225 x 70 x r16 but measure 28 x 9 x 17.5

Was hoping someone could explain, as i need a new set of tyres.

Thanks very much. Tom.

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Old 04-19-2016, 09:35 AM   #2
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Join Date: Feb 2012
Location: Cheshire Oregon
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31" is the height. 10" is the tread width. 15" is the rim size

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Old 04-19-2016, 10:03 AM   #3
Join Date: Jan 2016
Location: Central Point, Oregon
Posts: 165
There are two different standards for measurement - SAE and metric.

SAE sizes use Inches as the unit of measure. 31x10.50R15 defines a tire with a 31" "nominal" overall diameter, 10.5" nominal width, for a 15" rim. The diameter and width are "nominal" measurements and some brands/models do run a little small or a little big. The actual diameter/width on your vehicle can also be influenced by the width rim you run, air pressure, and weight of your vehicle. Generally speaking, I wouldn't worry about the variances. Most tires of a given size will "look" the same size, even if they're a fraction of an inch off. If you run an appropriate size for your lift height (or no lift) and wheel backspacing then this shouldn't be a concern.

Metric sizes use milimeters as the primary unit of measure. 265/70R16 defines a tire with a 265mm nominal width, a sidewall profile that is 70% of the width (~185.5mm sidewall) and uses a 16" rim. It's interesting that they mix metric tread width and a SAE rim size, but that's the way they do it in the USA. To get the overall diameter, take the width, multiply by the sidewall percentage, multiply by 2, convert to inches, and add the wheel diameter. For our example that would be 265*.7*2 = 371mm which is approximately 14.6". 14.6+16 = 30.6" overall diameter.

Clear as mud?
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