205/55R16 means a 205 mm wide tire, a sidewall 55% of that width tall, radial construction, for a 16-inch wheel. The 91V that can follow it adds the load index and the speed rating, and those two ratings are the tire working limits.

The code on the sidewall

Every tire carries its size moulded into the sidewall, in a short code that looks cryptic until it is read piece by piece, and then never looks cryptic again. The code 205/55R16 is the most common example, and it contains four facts. The first number, 205, is the section width in millimetres: the width of the tire from sidewall to sidewall at its widest point, measured on the rim it was designed for. The second number, 55, is the aspect ratio: the height of the sidewall as a percentage of the width. The letter R means radial construction, which is how almost every passenger tire today is built, with the cord plies running radially from bead to bead. The last number, 16, is the wheel diameter in inches: the size of the rim the tire is made to fit. Read in order, the code says a 205-millimetre-wide tire, with a sidewall 55% of that width tall, of radial construction, for a 16-inch wheel.

The aspect ratio is the piece most people misread, because it is a ratio, not a measurement. A 55 aspect ratio on a 205-millimetre tire is a sidewall of 112.75 millimetres, because 205 times 0.55 is 112.75. The same 55 on a 225-millimetre tire is a sidewall of 123.75 millimetres, which is taller, because the percentage is taken from a wider base. Two tires can share an aspect ratio and stand at different heights, and two tires can share a sidewall height with different aspect ratios, if their widths differ. The Tire Size Calculator does this multiplication for you, and the decode tool on the hub page shows the sidewall height in millimetres and inches for any size you type in. The habit to build is to read the aspect ratio as a relationship, never as a height on its own.

After the size, many tires carry a service description: a load index number and a speed letter, such as 91V in 205/55R16 91V. The two are always together, and the decode tool on the hub page requires both if either is entered, because a load index without a speed letter, or a speed letter without a load index, is an incomplete rating. The load index is the maximum load one tire can carry. The speed letter is the maximum sustained speed the tire is rated for. Together they are the tire’s working limits, and they matter more than the size alone, because a tire of the right size with too low a load index is overloaded from the moment the car is loaded, even though it looks correct.

Where to look: sidewall, placard, manual

The sidewall is the authoritative source for the tire that is actually fitted. Read it in daylight, on a clean tire, and read all four tires, because a car can carry a different size on one axle, and the spare is often a different size again, sometimes a temporary space-saver with its own limits printed on it. If the sidewall is worn, kerbed, or unreadable, do not guess from memory or from the tire’s appearance: move to the placard.

The placard is the sticker on the driver-side door jamb, sometimes inside the fuel-filler door or the glove box, that lists the recommended tire size and the recommended pressures for the car as sold. It is the maker’s recommendation for the car, and it is the size to compare any proposed swap against. It lists the size the car was designed around, including the load index and speed rating the maker specified. If the placard and the sidewall disagree, the car has been fitted with non-standard tires at some point, and a tire professional should confirm what fits before anything is bought, because the disagreement may be harmless, such as an approved optional size, or it may be a size that was never approved for the car.

The owner’s manual is the third source, and it answers a different question. It lists the approved sizes for the model, including optional sizes for different wheel packages, and it is the place to settle whether a size was ever approved for the car. It does not know what is fitted to your car today, so it cannot tell you the size of the tires you own. Between the three sources, the sidewall says what is fitted, the placard says what the maker recommends, and the manual says what else was approved. A size that appears in all three is certain. A size that appears in only one needs a professional check before it is trusted.

What no source on the car can do is look the size up from the registration number or the VIN on this site. The calculator has no vehicle database and no VIN lookup, and no page on this site will return a tire size from a number plate or a chassis number. That is a deliberate limit, not a missing feature: vehicle databases are maintained by makers and dealers, and a calculator that guessed from a model name would guess wrong for every car fitted with an optional wheel package. Read the size from the car itself, from one of the three places above, and type it in exactly as written, including the service description if it is present.

The load index, in full

The load index is a code number that stands for a weight in kilograms, and the code is not a round-number scale: each step adds a different amount, because the chart is built from the standard it follows rather than from arithmetic. The decode tool on the hub page reads the full chart, from index 60, which is 250 kg, to index 126, which is 1,700 kg. The table below carries the range most passenger cars use, so the common values can be checked at a glance without a tool. The weight is per tire, not per car: a car with four tires rated at 615 kg each carries a maximum of 2,460 kg across the four, and the load must also respect the axle limits on the placard, because a car is rarely loaded evenly across its axles.

The rule that matters is simple: never fit a replacement with a lower load index than the placard calls for. A lower index overloads the tire even when the car looks fine standing still, because braking, cornering, and bumps all add load beyond the static weight, and the rating already accounts for that margin. A higher index is acceptable in itself, though it may come with a stiffer ride, because a tire built to carry more is usually built more heavily. The index is also the reason the size alone is never enough: two tires of identical size can carry different loads, and the one with the lower index is the one that will fail first under a heavy car.

The speed rating, in full

The speed letter is the maximum sustained speed the tire is rated for, under its rated load, and the letters do not run in alphabetical order at the top of the scale, which is the second most common misreading on a sidewall. The standard ratings run L at 120 km/h, M at 130, N at 140, P at 150, Q at 160, R at 170, S at 180, T at 190, U at 200, H at 210, V at 240, W at 270, and Y at 300. The jump from H to V skips the letters that would be expected, for historical reasons, and the decode tool on the hub page accepts exactly the standard set, from L to Y, and rejects anything outside it.

The rule for speed ratings matches the rule for load: do not fit a lower rating than the original tire carried. A tire rated for 190 km/h that is driven at motorway speeds is operating inside its rating, but a tire rated for 160 km/h fitted to a car built for 210 km/h is operating outside the rating the maker specified, and the rating is about sustained heat build-up, not about a single burst of speed. A higher rating is acceptable, and it is common on replacement tires, because a higher-rated tire is built to manage more heat. The rating does not make a car faster, and it does not change the legal speed limit, which is set by the road, not by the tire. It is a safety margin, and margins should not be spent without a reason.

Reading real sidewalls: five examples

Example one: 205/55R16 91V. Width 205 mm, aspect ratio 55, radial, 16-inch rim, load index 91, which is 615 kg per tire, speed rating V, which is 240 km/h. This is the default example on the hub page, and it is the common family hatchback size, which is why it is used as the reference throughout this guide.

Example two: 225/65R17 102H. Width 225 mm, aspect ratio 65, radial, 17-inch rim, load index 102, which is 850 kg per tire, speed rating H, which is 210 km/h. The taller sidewall, 146.25 mm against 112.75 mm for the first example, is the mark of a larger vehicle, and the higher load index matches the heavier car. The overall diameter is 724.3 mm, which is why the two tires look nothing alike standing side by side, even though both are ordinary road tires.

Example three: 195/65R15 91T. Width 195 mm, aspect ratio 65, radial, 15-inch rim, load index 91, which is 615 kg, speed rating T, which is 190 km/h. A smaller, older size, still common, and a reminder that the same load index appears on many sizes: the index is about the tire’s construction, not about its dimensions.

Example four: 265/70R17 115T. Width 265 mm, aspect ratio 70, radial, 17-inch rim, load index 115, which is 1,215 kg per tire, speed rating T, which is 190 km/h. A large truck and SUV size, with a sidewall of 185.5 mm and an overall diameter of 802.8 mm. The load index is the headline here: this tire carries nearly twice the load of the family car tire in example one, which is exactly what the heavier vehicle needs.

Example five: 225/45R17 94W. Width 225 mm, aspect ratio 45, radial, 17-inch rim, load index 94, which is 670 kg, speed rating W, which is 270 km/h. A low-profile performance size, with a short 101.25 mm sidewall and a high speed rating. The short sidewall sharpens steering response and transmits more road harshness, which is the trade the size makes, and the speed rating is the construction that makes it safe at the speeds the car is built for.

Mistakes when reading a size

Mistake one: reading the aspect ratio as a height in millimetres. A 55 aspect ratio is not 55 mm of sidewall on any tire, because it is a percentage of the width. Mistake two: reading the rim number as the tire’s diameter. The 16 in 205/55R16 is the wheel diameter, and the tire’s overall diameter is the wheel plus two sidewalls, which is 631.9 mm for this size, not 16 inches. Mistake three: dropping the service description, and fitting a tire of the right size with too low a load index or speed rating. Mistake four: reading one tire and assuming all four match. Mistake five: trusting a size remembered from a previous car, or from a website that lists sizes by model, when the car in front of you may carry an optional wheel package. Every one of these mistakes is prevented by the same habit: read the sidewall, read the placard, and type exactly what is written.

Frequently asked questions

How do I read a tire size like 205/55R16?

205 is the section width in millimetres, 55 is the aspect ratio, the sidewall height as a percentage of the width, R means radial construction, and 16 is the wheel diameter in inches. The sidewall height is 205 times 0.55, which is 112.75 mm, and the overall diameter is the wheel plus two sidewalls, which is 631.9 mm. The number and letter that can follow, such as 91V, are the service description: load index and speed rating.

Where is my tire size written?

On the tire sidewall first, moulded into the rubber, and on the driver-side door jamb placard second, which lists the recommended size and pressures for the car as sold. The owner’s manual lists the approved sizes for the model. The sidewall says what is fitted, the placard says what the maker recommends, and the manual says what else was approved. Check all four tires, because sizes can differ between axles.

What does the 91V after a tire size mean?

91 is the load index, the maximum load one tire can carry, which is 615 kg for a 91. V is the speed rating, the maximum sustained speed the tire is rated for, which is 240 km/h for a V. Never fit a replacement with a lower load index than the placard calls for, and do not fit a lower speed rating than the original tire carried.

Can I look up my tire size from my car model or VIN?

Not on this calculator. It has no vehicle database and no VIN lookup, and it needs the size typed in exactly as written on the sidewall or placard. A model name alone is not enough, because cars are sold with optional wheel packages, and a size guessed from a model list can be wrong for the car in front of you.

Is the aspect ratio the sidewall height?

No. The aspect ratio is the sidewall height as a percentage of the width, not a measurement on its own. A 55 aspect ratio on a 205 mm tire is 112.75 mm of sidewall, and the same 55 on a 225 mm tire is 123.75 mm. Always multiply the width by the aspect ratio to get the sidewall height, or let the decode tool on the hub page do it.

What if the sizes on my car do not match the placard?

The car may have been fitted with non-standard tires, or with an approved optional size. Do not assume either. Have a tire professional confirm what fits before buying, and compare the fitted size with the approved sizes in the owner’s manual. The comparison tool on the hub page can check the diameter difference, but it cannot confirm clearance or rim fitment.