Start with the size, and read it left to right
The size is the string that looks like 225/65R17. Read it as four separate facts rather than one number. 225 is the section width in millimetres, measured sidewall to sidewall at the widest point - not the tread width, which is narrower. 65 is the aspect ratio: the sidewall is 65 percent of the section width, so 146.25 mm here. R is the construction, radial in essentially every modern passenger tire. 17 is the rim diameter in inches.
That the width is metric and the rim is imperial in the same string is not a mistake. The two numbers come from different measuring traditions and were never reconciled, which is why every dimension anyone wants from a tire size involves a conversion somewhere.
Then the prefix, or the absence of one
A letter before the width says which family the tire belongs to and, more importantly, which standards body's load tables rate it. P is P-metric. LT is light truck. ST is a trailer tire. T is a temporary spare. A C after the rim diameter marks a commercial van size.
No prefix at all is a reading in its own right: it makes the tire Euro-metric. A P-metric tire and a Euro-metric tire of identical nominal size are rated by different tables and can carry different loads at the same pressure. Treating a missing P as a typographical detail is the most expensive misreading on the sidewall.
Then the service description
After the size comes something like 102H. The number is the load index, a code that maps to a maximum load for one tire in a published table. The letter is the speed symbol, another code that maps to a maximum speed. Both are codes, not measurements: 102 is not 102 of anything.
Two things go wrong here. The speed table is not alphabetical, so H is 210 km/h and sits between U and V rather than down with G and J. And brackets are meaningful: a tire marked (Y) was tested above 300 km/h, not at it. Reading (Y) as Y understates the tire.
Then the DOT code, which is the only marking that dates the tire
Find DOT and read to the end of that string. The last four digits are the week and year of manufacture: 2519 is the 25th week of 2019. Three digits mean the tire was built before 2000, and the code carries only one digit of the year, so the decade is not recoverable from the tire at all.
The date code is only required on one sidewall. Finding DOT with nothing after it means you are looking at the wrong side, not at an unmarked tire.
Everything else is a separate system
The UTQG grades - a treadwear number and two letters - are set by a federal regulation and measure narrower things than their names suggest. The load range letter on a light truck tire describes construction strength and the pressure the tire holds. M+S and the three peak mountain snowflake are winter markings, and only one of them was earned by a test.
- Treadwear is a comparison against a reference tire on one test course, self-reported by the manufacturer. It is not a mileage.
- Traction grades measure straight ahead braking on wet asphalt and wet concrete. Nothing else.
- Temperature grades measure heat resistance on an indoor test wheel, assuming correct inflation and no overloading.
- M+S is awarded on tread geometry alone. The snowflake requires 110 percent of a reference tire's traction on medium packed snow.