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Just wondering about this, unless the R doesn't mean "Radius" it doesn't make much sense.

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  • 32
    R is of course for round as opposed to S16 square wheels ;)
    – jpa
    Commented Jan 26, 2017 at 6:10

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Tires (or tyres) are marked as www/hhRdd where www is the tread width in mm, hh is the sidewall height as a percentage of width and dd is the inner diameter in inches. I.e. 245/75R16 has a width of 24.5cm, 75% (~18cm) height and 16in (~40cm) diameter. The R indicates the construction of the tire is 'radial' ply. 'Bias ply' tires, which were common before radials became the standard are indicated with a blank space. Other tire constructions exist, but are rarely used on highway vehicles.

Points to anyone who can give a good reason for mixing three different systems of measurement in a single size.

"The code also usually has a prefix indicating whether it for a car, truck, etc - also there seems to often be lots of non-standard suffixes for manufacturer date, load & speed ratings etc" -Wilf

Great point. Much more information about tyre sizes can be found here: http://www.tirebuyer.com/education/understanding-tire-sizes-and-types

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    At a guess, default rim diameters were established somewhere in the middle of the 20th century, before metrification became the norm. There's nothing stopping anyone making metric-sized rims, except there are no tyres because there are no metric rims. Same reason bicycle chain is 1/2" pitch, because that is simply what it is.
    – Criggie
    Commented Jan 26, 2017 at 8:43
  • The code also usually has a prefix indicating whether it for a car, truck, etc - also there seems to often be lots of non-standard suffixes for manufacturer date, load & speed ratings etc
    – Wilf
    Commented Jan 26, 2017 at 14:03
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    I really wish manufacturers simplified their sidewall height to a static measurement. It would certainly make things easier when trying to maintain overall circumference while going with wider tires for, say, an AWD vehicle. Instead, I have to math and then determine if the height difference is an acceptable tolerance or not.
    – Ellesedil
    Commented Jan 26, 2017 at 19:54
  • @Criggie: well, not really true, there are: Michelin TRX (and related). Not very widespread any more but it existed and still exists.
    – Gábor
    Commented Jan 27, 2017 at 11:26
  • @Criggie it's too bad default rim diameters weren't established at the turn of the 20th century. Still trying to find some 75/85R30s to put on my Infiniti
    – Trevor D
    Commented Jan 27, 2017 at 18:16

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