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 the size actually promisesThe load index is a code into a standardised series, not a weight. How the series is spaced, why four tires do not simply add up to your vehicle's rating, and how dual-wheel indices are printed.
- 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 a speed symbol is a statement about a laboratory test rather than about a road.
- DOT date codeReading a DOT code: the week, the year and the plantThe last four digits of the Tire Identification Number are a week and a year. What the rest of the code holds, why the standard length changed to thirteen symbols, and why one sidewall usually 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, what the ±3% tolerance is really about, and the arithmetic of going the other way for winter.
- Wheel offset and backspacingWheel offset and backspacing: one geometry, two unitsOffset is millimetres from the mounting face to the wheel's centreline; backspacing is inches from the mounting face to the inner flange. The conversion, and why a wider wheel at the same offset moves the tire both ways at once.
- Rim width to tire widthRim width changes tire width, and the size code cannot knowSection 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 small loss of sidewall height nobody accounts for.
- Revolutions per mileRevolutions per mile does not move in step with diameter63,360 divided by the tire's circumference. The figure your ABS sensors, odometer and gearing arithmetic all actually use — and the reason a 10% taller tire turns 9% fewer times, not 10%.
- Aspect ratioAspect ratio, and why the same number means different thingsThe middle number of a tire size is a multiplier applied to the first one, so 70-series sidewalls on a 225 and a 315 differ by two and a half inches. What that does to diameter, in figures.
- Axle ratio basicsAxle ratios: tooth counts, tire diameter and restoring rpmWhat a 3.73 axle actually counts, why a bigger tire behaves like a numerically lower one, and how to compute the ring-and-pinion ratio that puts engine speed back where it started.
- Transmission gear ratiosTransmission ratios, overall ratio and crawl ratioThe gearbox multiplies before the axle does. How to chain the ratios into one overall figure, what a transfer case low range does to it, and why an eight-speed changed the whole regearing conversation.
- XL and Reinforced markingsXL, Extra Load and Reinforced: one marking, three spellingsAn 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 — which it only delivers at that pressure.
- Load range and ply ratingLoad range letters and the ply ratings they replacedLoad Range E does not mean ten plies. The letters encode a reference inflation pressure — 50, 65, 80, 95 psi — and everything else about the casing follows from that.
- M+S and 3PMSF markingsM+S and the three-peak snowflake are two different claimsOne is a self-declared tread pattern description, the other is a passed test against a reference tire on packed snow. What each marking certifies, what neither covers, and why both appear on the same sidewall.
- UTQG treadwear, traction and temperatureUTQG grades: what treadwear, traction and temperature meanThe three-part grade moulded on a passenger tire's sidewall. Treadwear is a manufacturer's own projection against a control tire, traction is a wet skid coefficient with published thresholds, temperature is a speed the tire survived.
- Construction codes: R, D and BR, D and B: the construction letter in a tire sizeThe single letter between the aspect ratio and the wheel diameter states how the casing is built. Radial, diagonal bias and bias-belted are three different structures, and the letter is not decoration.
- Section width vs tread widthSection width, tread width and contact patch: three widthsThe number in the size code measures the widest point of the casing, which is usually the sidewall. What touches the road is narrower, and how much of it touches is set by load and pressure rather than by either figure.
- Overall diameter and rolling radiusOverall diameter, loaded radius and rolling radius differThe 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 by twice its tread depth — here is which number applies to what.
- How a speedometer is calibratedSpeedometer calibration, and what a tire change does to itThe reading is a stored constant applied to wheel pulses. Change rolling circumference and the whole instrument shifts by the same ratio — including the odometer. How to measure the error and what can be recalibrated.
- Staggered fitmentsStaggered fitments: different sizes front and rearWhen the rear tires are wider than the front, the rear tires are usually taller too — because sidewall height is a percentage of width. What that costs in rotation, spares and instrument calibration.
- Spare tire sizesSpare tire sizes: the temporary turns 195 more times a 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, and why the markings on a compact spare say what they say.
- Pressure and load tablesLoad and inflation tables: how pressure buys capacityA tire's load index is stated at one pressure. The manufacturer's table gives every other pressure, and reading it is how you find the inflation a substituted size needs to match what your placard asked for.
- Uniformity dots and balance marksRed dots, yellow dots, and what a fitter does with themThe coloured marks on a new tire's sidewall are mounting instructions. Red marks the point of maximum radial force variation, yellow the lightest point, and they are used in two mutually exclusive ways.