Tire size (P215/65R15)

How to read it

US federal rules require the size designation on each sidewall of a car or light truck tire, written as it is listed in the tire and rim standards: the Tire and Rim Association’s, the European Tyre and Rim Technical Organization’s (ETRTO) or the Japanese tire makers’ association’s (FMVSS 139). Ford’s owner’s manual takes P215/65R15 95H as its example. Read left to right: P is a passenger car tire; 215 is the nominal width in millimetres, from sidewall edge to sidewall edge; 65 is the aspect ratio, the sidewall’s height as a percentage of that width; R is radial construction; and 15 is the diameter of the wheel it fits, in inches. The 95H after the size is not part of it. Bridgestone calls it the service description: the Load index (95 in 95H) (95) and the Speed rating (H in 95H) (H).

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Sizes with another letter, or none

A size with no letter in front, such as 205/55R16, is a Euro-metric size, designed to ETRTO’s standard; Ford’s manual says it may instead be a size designated by the Japanese makers’ association. Yokohama says Euro-metric sizes are dimensionally equivalent to P-metric sizes but typically differ subtly in how much they can carry.

LT in front marks a light truck tire, T a temporary spare and ST a trailer tire. Yokohama describes two sizes where the letters come at the end: older light truck flotation sizes such as 35x12.50R17LT, and Euro-metric sizes ending in C, such as 195/70R15C, which mark a commercial tire for vans and delivery trucks.

Other letters in place of R

ZR in place of R, as in P275/40ZR17, marks a tire for speeds above 240 km/h (149 mph). D marks diagonal (bias ply) construction; Australia’s design rule for passenger tyres also lists B, for bias-belted. Yokohama notes that ETRTO lets makers add an F to the size to mark a self-supporting Run-flat markings (RFT, ROF, ZP, SSR) tire.

Where else the size is printed

The vehicle’s tire placard, fixed to the driver’s side door pillar, gives the size of the tires fitted when the vehicle was new, under the heading “size” or “original tire size”, and the size of the spare (FMVSS 110). The same rule lets the maker print the load index and speed rating beside it. For cars and light trucks made since September 2005, federal rules also require the owner’s manual to explain each marking on the tires supplied with the vehicle (49 CFR 575.6).