Reference
The sidewall is a specification sheet moulded into rubber. This section reads it out.
A tire carries more printed information than almost anything else you own, and most of it is ignored because there is no obvious place to look it up. These pages are that place.
They are organised by what you are looking at rather than by category. If you are staring at a number you do not recognise, the size-code page names every element in order. If you are comparing two tires that look identical on paper, the load and speed pages are where the real difference usually is.
What is here
The size code and its parts: section width, aspect ratio, construction letter, wheel diameter, and the service-type prefix that changes what a tire is rated for without changing its shape.
The ratings: load index, speed symbol, load range and ply rating, and the Extra Load marking that raises a tire's capacity without changing its size.
The markings: the date code that tells you how old a tire is, the M+S and three-peak snowflake symbols and the very different things they mean, and the treadwear, traction and temperature grades.
The geometry: aspect ratio, revolutions per mile, overall diameter and rolling radius, section width against tread width, plus-sizing, and how rim width changes the tire mounted on it.
The drivetrain side: axle ratios, transmission gear ratios, and how a speedometer is calibrated in the first place — which is what makes the error arithmetic work.
On spelling
These pages use the American spelling, tire. The British spelling is tyre; the two words mean exactly the same thing, and both appear on this page so that searching for either finds it. The markings themselves are internationally standardised and identical either way — a tyre bought in Manchester and a tire bought in Michigan carry the same size code, the same load index and the same speed symbol, in the same order.
The three things worth reading first
If you read nothing else here, read these.
The size code, digit by digit. Once you can decompose a code, every other number on the site becomes derivable rather than mysterious, and the aspect ratio stops being the trap it is for most people.
Load index. It is the marking most likely to matter and least likely to be checked. Two tires with the same size code can differ by hundreds of pounds of capacity, and the difference is a number the size does not mention.
Revolutions per mile. Everything downstream — speedometer, odometer, engine speed, effective final drive — is derived from it. Understanding that one figure collapses four separate questions into one.
What makes these pages different from the calculators
The calculators answer a question you already have. These pages are for the moment before that, when you are looking at a marking and do not yet know what question to ask.
They are written to be read out of order and in fragments, because that is how anybody actually uses a reference: standing next to a tire, looking up one thing. Each page assumes you arrived directly and explains its own terms rather than assuming you read the others first.
Where a page carries a table — load index, speed symbols — the table is generated from the same data the decoder tool reads, not retyped into the page. A sixty-row table transcribed by hand is sixty chances to be wrong, and a second copy of numbers that already exist is free to drift from the first the moment either is edited.
- The tire size code, digit by digitThe tire size code, digit by digitEvery element of a tire size marking explained in order — service type, section width, aspect ratio, construction letter and wheel diameter, in both notations.
- Load index tableLoad index: what the number after a size meansThe load index is a code into a standardised series, not a weight. How the series is spaced, and why four tires do not add up to your vehicle's rating.
- Speed rating tableSpeed symbols: an alphabet that is not in orderWhat the letter after the load index certifies, why H sits between U and V, what happened to Z, and why the symbol describes a laboratory test.
- DOT date codeReading a DOT code: week, year and plantThe last four digits of the Tire Identification Number are a week and a year. What the rest holds, and why one sidewall carries less of it than the other.
- Metric vs imperial notationConverting between metric and inch tire sizesHow to turn 315/70R17 into inches, why the answer is 34.36 and not 35, and what to do about the fact that the inverse conversion has more than one right answer.
- Plus sizingPlus sizing: bigger wheel, same rolling diameterThe convention that trades sidewall for wheel while holding overall height. How to compute the replacement aspect ratio, and what the ±3% tolerance is about.
- Wheel offset and backspacingWheel offset and backspacing: two unitsOffset is millimetres from the mounting face to the wheel's centreline; backspacing is inches from that face to the inner flange. The conversion between them.
- Rim width to tire widthRim width changes tire width; the code cannotSection width is specified on one nominal measuring rim. Mount the same tire half an inch wider and it gains roughly 0.2 in across, plus a loss of height.
- Revolutions per mileRevolutions per mile is not linear in diameter63,360 divided by the tire's circumference. The figure your ABS sensors, odometer and gearing arithmetic all use — and why a 10% taller tire turns 9% fewer.
- Aspect ratioAspect ratio: why one number means two thingsThe middle number of a tire size is a multiplier applied to the first, so 70-series sidewalls on a 225 and a 315 differ by two and a half inches.
- Axle ratio basicsAxle ratios: tooth counts and restoring rpmWhat a 3.73 axle actually counts, why a bigger tire behaves like a numerically lower one, and how to compute the ratio that restores engine speed.
- Transmission gear ratiosTransmission, overall and crawl ratiosThe gearbox multiplies before the axle does. How to chain the ratios into one overall figure, and what a transfer case low range does to the result.
- XL and Reinforced markingsXL, Extra Load, Reinforced: one markingAn XL tire is the same size as its standard-load twin. What it has is a higher reference inflation, and therefore a higher load index at that pressure.
- Load range and ply ratingLoad range letters and old ply ratingsLoad Range E does not mean ten plies. The letters encode a reference inflation pressure — 50, 65, 80, 95 psi — and the casing follows from that.
- M+S and 3PMSF markingsM+S and the snowflake: two different claimsOne is a self-declared tread pattern description, the other a passed test on packed snow. What each marking certifies, and what neither of them covers.
- UTQG treadwear, traction and temperatureUTQG: treadwear, traction and temperatureThe three-part grade moulded on a passenger tire's sidewall: treadwear is a manufacturer's own projection, traction a wet skid coefficient, temperature a speed.
- Construction codes: R, D and BR, D and B: the construction letterThe single letter between the aspect ratio and the wheel diameter states how the casing is built. Radial, diagonal bias and bias-belted are three structures.
- Section width vs tread widthSection width, tread width, contact patchThe number in the size code measures the widest point of the casing, usually the sidewall. What touches the road is narrower, and set by load and pressure.
- Overall diameter and rolling radiusOverall diameter, loaded and rolling radiusThe diameter computed from a size code is a free, unloaded figure. Under load the tire squats, under speed it grows, and over its life it shrinks.
- How a speedometer is calibratedSpeedometer calibration and a tire changeThe reading is a stored constant applied to wheel pulses. Change rolling circumference and the whole instrument shifts by the same ratio, odometer included.
- Staggered fitmentsStaggered fitments: front and rear differWhen the rear tires are wider than the front they are usually taller too, because sidewall is a percentage of width. What that costs in rotation and spares.
- Spare tire sizesSpare tire sizes: 195 more turns per mileA T125/80D16 stands 23.87 in against a full-size tire's 31.06 in — a 23% difference in rolling diameter. What that does to the driveline.
- Pressure and load tablesLoad and inflation: how pressure buys loadA tire's load index is stated at one pressure. The manufacturer's table gives every other, and reading it is how a substituted size matches your placard.
- Uniformity dots and balance marksRed dots, yellow dots and what they markThe coloured marks on a new tire's sidewall are mounting instructions. Red marks maximum radial force variation, yellow the lightest point, used two ways.