Every tire carries its specification moulded into its sidewall, in a code that looks cryptic and is in fact perfectly systematic: three numbers, a letter, two more numbers, and two more codes after them. The code states the tire’s width, the height of its sidewall as a percentage of that width, its construction, the diameter of the wheel it fits, and how much weight it can carry at what speed. Reading it is the difference between buying the right tire and buying a tire that fits the wheel but not the car — and the difference matters, because a tire of the wrong diameter misreports speed and distance, a tire of the wrong load index is unsafe at full load, and a tire of the wrong speed rating is illegal in some jurisdictions at the car’s design speed.

Reading the code: 205/55R16 91V, position by position

Take 205/55R16 91V, a common size. 205 is the section width — the tire’s widest point, sidewall to sidewall — in millimetres. 55 is the aspect ratio: the sidewall height expressed as a percentage of the section width, so the sidewall of this tire is 205 × 0.55 = 112.75 mm tall. R denotes radial construction, in which the ply cords run radially across the tire; virtually every passenger tire today is radial, and the letter is a legacy of the bias-ply era it replaced. 16 is the rim (wheel) diameter in inches — the one imperial number in a metric code, and the reason the code mixes units. After the size come the service description: 91, the load index, and V, the speed rating.

Further marks follow on the sidewall. A letter before the width (P for passenger, LT for light truck) is the US service type and is often omitted outside North America. XL or RF marks an extra-load (reinforced) tire, with a higher load capacity at higher pressure. M+S marks mud and snow capability; the three-peak mountain snowflake symbol marks a tire that meets the severe snow service standard. The DOT code ends in a four-digit date: the week and year of manufacture — 2324 is the 23rd week of 2024 — and a tire’s age counts from that date, not from the date it was fitted.

Diameter: the number everything else depends on

Overall diameter = (2 × sidewall height) + rim diameter. For 205/55R16: sidewall 112.75 mm, doubled 225.5 mm, plus a 16-inch rim (406.4 mm) = 631.9 mm overall. The diameter sets the tire’s circumference (π × diameter = 1,985 mm), and the circumference sets how far the car travels per revolution — which is why the diameter governs the speedometer and odometer. A tire 3% larger in diameter travels 3% farther per revolution, so at a true 100 km/h the speedometer (calibrated for the original tire) reads about 97 km/h, and the odometer under-records distance by the same 3%. A tire 3% smaller over-reads by 3% and accumulates distance faster than the car actually travels.

The ±3% window is the industry rule of thumb for replacement sizes, and it exists because the effects compound: beyond it, gearing changes (the engine turns fewer or more revolutions per kilometre), ground clearance changes (a larger tire lifts the car, a smaller one lowers it), and the tire may foul the wheel arch or suspension at full lock or full compression. The window is a guideline, not a law — the vehicle manufacturer’s approved sizes, listed on the door jamb or in the handbook, are the authority — but any size outside the window needs a reason, and “it was cheaper” is not one.

Plus-sizing: wider, lower, same diameter

Plus-sizing fits a larger wheel with a lower-profile tire, keeping the overall diameter the same. The worked example: 205/55R16 (diameter 631.9 mm) to 225/45R17 — width 225 mm, aspect ratio 45, sidewall 225 × 0.45 = 101.25 mm, doubled 202.5 mm, plus a 17-inch rim (431.8 mm) = 634.3 mm, a difference of +0.38%, inside the window. The wider tire puts more rubber on the road, the shorter sidewall flexes less in corners, and the larger wheel fills the arch — at the cost of ride comfort (less sidewall to absorb bumps), cost (larger wheels and tires), vulnerability to pothole damage, and usually higher rolling resistance.

Plus-sizing changes the section width too, and the width must suit the rim: a 225 mm tire needs a rim roughly 7 to 8.5 inches wide, and fitting it to a rim outside the approved range distorts the tread profile — too narrow a rim pinches the tread into a crown, too wide stretches it flat — and both distort the contact patch the plus-size was bought to improve. The tire-size calculation on this site compares diameters, circumferences, sidewalls and speedometer error across sizes, so a plus-size can be checked against the 3% window before it is bought, not after it is fitted.

Load index and speed rating: the two codes that carry safety

The load index is a code for the maximum weight one tire may carry at its rated pressure: 91 means 615 kg, 94 means 670 kg, 100 means 800 kg, and the table runs from the 60s to beyond 120 for light trucks. The index on the tire must meet or exceed the vehicle manufacturer’s specification — the figure on the door jamb, derived from the vehicle’s gross axle weights divided by the tires per axle, with a margin. Fitting a lower-index tire than specified is unsafe at full load and can void insurance; fitting a higher-index tire is safe but usually rides harder, because the stiffer construction that carries more weight flexes less.

The speed rating is the maximum sustained speed the tire is certified for: S is 180 km/h, T 190, H 210, V 240, W 270, Y 300, with ZR covering the highest-performance ratings. The rating must meet or exceed the vehicle’s design maximum speed as specified by the manufacturer, and in several European jurisdictions it is a legal requirement, not a recommendation. Winter tires are the recognised exception, with a lower speed rating permitted and a sticker on the dashboard reminding the driver of the limit. A tire that matches the size code but not the load and speed codes is the wrong tire, however well it fits the wheel.

Metric, imperial, and the codes that mix them

The size code mixes systems deliberately: width in millimetres, rim diameter in inches, aspect ratio as a percentage. The mix is historical — the rim standard was set in inches and never converted — and it is the source of the commonest conversion error, which is treating the rim number as millimetres. 16 means 16 inches (406.4 mm), and a 205/55R16 tire does not fit a 16 mm anything. Older codes add variants: a size written 205/55ZR16 carries the same dimensions with a Z speed rating embedded, and flotation sizes for light trucks (31×10.50R15) state the overall diameter in inches first, then the width in inches — a different order, for a different market, describing the same three quantities.

Frequently asked questions

What does 205/55R16 mean?

205 is the section width in millimetres, 55 is the aspect ratio (the sidewall height is 55% of the width, or 112.75 mm), R means radial construction, and 16 is the rim diameter in inches. After the size, 91V would be the load index (91 = 615 kg) and the speed rating (V = 240 km/h). Overall diameter is (2 × 112.75 mm) + 406.4 mm = 631.9 mm.

How much can I change my tire size?

Keep the overall diameter within ±3% of the original tire. Outside that window the speedometer and odometer misreport, gearing changes, and clearance can suffer. The vehicle manufacturer’s approved sizes, on the door jamb or in the handbook, are the authority — the 3% window is the guideline for sizes not listed there.

What is plus-sizing?

Fitting a larger wheel with a lower-profile tire while keeping the overall diameter the same — for example 205/55R16 to 225/45R17, a difference of +0.38%. The wider tire and shorter sidewall improve cornering response at the cost of ride comfort, price, and pothole vulnerability. The new width must suit the rim width, or the tread profile distorts.

What does the load index mean?

It is a code for the maximum weight one tire may carry: 91 means 615 kg, 94 means 670 kg, 100 means 800 kg. The index fitted must meet or exceed the vehicle manufacturer’s specification, derived from the gross axle weights. A lower-index tire is unsafe at full load; a higher-index tire is safe but usually rides harder.

What does the speed rating letter mean?

It is the maximum sustained speed the tire is certified for: S 180 km/h, T 190, H 210, V 240, W 270, Y 300. The rating must meet or exceed the vehicle’s design maximum speed as specified by the manufacturer, and in several European jurisdictions it is a legal requirement. Winter tires are the recognised exception, with a dashboard sticker stating the limit.

How old is my tire?

Read the DOT code on the sidewall: the last four digits are the week and year of manufacture — 2324 is the 23rd week of 2024. Age counts from manufacture, not from fitting. Most manufacturers recommend inspection after five years and replacement at ten, whatever the tread depth, because the rubber compounds degrade with age.