Tyre Size Decoder

About Tyre Size Decoder

How Tyre Size Codes Work

The tyre size marking system used worldwide follows standards set by the European Tyre and Rim Technical Organisation (ETRTO) and the Tire and Rim Association (TRA). A typical passenger car tyre code has this structure:

[Width] / [Aspect Ratio] [Construction] [Rim Diameter] [Load Index] [Speed Rating]

Taking 215/55R17 91V as a worked example:

Code Part	Value	Meaning
215	215 mm	Section width - the distance across the tread from sidewall to sidewall when mounted and inflated
55	55%	Aspect ratio - the sidewall height is 55% of the section width. So 215 x 0.55 = 118.25 mm sidewall height
R	Radial	Internal construction type. R = radial (standard for cars), D = diagonal/bias, B = belted bias
17	17 inches	Rim diameter - the wheel size this tyre fits
91	615 kg	Load index - maximum weight per tyre at the correct inflation pressure (per ETRTO tables)
V	240 km/h	Speed rating - maximum sustained speed the tyre is designed for

Some tyres include a letter prefix before the width. P indicates P-metric (passenger car, common in North America), LT means light truck, and ST means special trailer. European-market tyres typically omit the prefix entirely. The decoder handles all of these formats.

The section width is measured in millimetres and represents the widest point of the tyre from outer sidewall to outer sidewall when mounted on the recommended rim width and inflated to the correct pressure. It is not the tread width, which is slightly narrower. Common widths range from 155 mm on small city cars up to 335 mm on wide-body sports cars.

The aspect ratio is one of the most misunderstood numbers. It is not a fixed height in millimetres. It is the sidewall height expressed as a percentage of the section width. A 55-series tyre on a 215 mm width has a 118.25 mm sidewall. The same 55 on a 245 mm width gives a 134.75 mm sidewall. This is why you cannot just swap widths without recalculating the overall diameter.

The construction letter is almost always R for radial. Radial tyres have their internal cord plies running perpendicular to the direction of travel, which gives better road contact, more even tread wear, and lower rolling resistance compared to the older diagonal (bias-ply) construction. Bias-ply tyres (marked D) are now mainly found on trailers, vintage vehicles, and some agricultural equipment. Belted bias (B) adds stabiliser belts to a bias carcass and is rare in modern production.

Calculating Tyre Dimensions from the Code

Two of the most useful measurements you can derive from the code are the sidewall height and the overall diameter of the tyre when mounted.

Sidewall height = Section width x (Aspect ratio / 100)

For a 215/55R17: 215 x 0.55 = 118.25 mm

Overall diameter = (Rim diameter in mm) + (2 x Sidewall height)

The rim diameter needs converting from inches to millimetres first (multiply by 25.4). So: (17 x 25.4) + (2 x 118.25) = 431.8 + 236.5 = 668.3 mm, which is about 26.3 inches.

Circumference = pi x Overall diameter = 3.14159 x 668.3 = 2,099.3 mm, or about 2.1 metres per revolution.

Revolutions per mile = 1,609,344 / Circumference = 1,609,344 / 2,099.3 = approximately 767 revolutions per mile.

These derived numbers matter in practice. Knowing the overall diameter is important when you are considering total cost of ownership for a vehicle, because tyre size affects fuel consumption, ride comfort, and replacement cost. A lower-profile tyre (smaller aspect ratio) generally means better handling but a firmer ride and more vulnerability to pothole damage. A higher-profile tyre absorbs bumps better but can feel less precise in corners.

The overall diameter also determines your speedometer accuracy. Your car's speedometer is calibrated for a specific number of wheel revolutions per distance. If you fit tyres with a significantly different diameter from the factory specification, your speedometer will read incorrectly. A tyre that is 3% larger in diameter means your true speed is 3% higher than what the dashboard shows. As a rule of thumb, keep within 3% of the original overall diameter when changing sizes.

Here are some common tyre sizes and their calculated dimensions for comparison:

Tyre Size	Sidewall (mm)	Overall Diameter (mm)	Overall Diameter (inches)
195/65R15	126.8	634.5	25.0
205/55R16	112.8	632.0	24.9
215/55R17	118.3	668.3	26.3
225/45R18	101.3	659.4	26.0
235/40R19	94.0	670.6	26.4
245/35R20	85.8	679.5	26.8

Notice how different width/profile/rim combinations can produce similar overall diameters. A 205/55R16 and a 195/65R15 are within 3 mm of each other, which is why manufacturers often list both as approved alternatives.

Load Index and Speed Rating Explained

The load index is a number from 60 to 120 (for standard passenger tyres) that maps to a maximum weight in kilograms per the ETRTO standard. This is the load per tyre at the manufacturer's recommended inflation pressure. Multiply by four for the total vehicle capacity. Some SUVs and vans use "extra load" (XL) or "reinforced" (RF) tyres that have a higher load capacity for the same index number when inflated to a higher pressure.

Common load index values and where you will find them:

Load Index	Max Load (kg)	Max Load (lbs)	Typical Application
71	345	761	Small city cars (e.g. Fiat 500)
75	387	853	Superminis (e.g. VW Polo, Ford Fiesta)
85	515	1,136	Medium hatchbacks (e.g. VW Golf, Ford Focus)
91	615	1,356	Family saloons (e.g. BMW 3 Series, Audi A4)
95	690	1,521	Large saloons and estates
100	800	1,764	Large SUVs and crossovers
110	1,060	2,337	Vans and light commercial vehicles
120	1,400	3,086	Heavy-duty vans and light trucks

Speed ratings use letters from L (120 km/h) through Y (300 km/h). The sequence is not perfectly alphabetical - H (210 km/h) sits between U and V for historical reasons. The Z rating originally meant "above 240 km/h" and can appear either as ZR in the construction position or as a standalone speed symbol. Key ratings for reference:

Rating	Max Speed	Common Vehicles
Q	160 km/h (100 mph)	Winter tyres, SUVs
S	180 km/h (112 mph)	Family saloons, MPVs
T	190 km/h (118 mph)	Family saloons, estate cars
H	210 km/h (130 mph)	Sports saloons, hot hatches
V	240 km/h (149 mph)	Sports cars, GTs
W	270 km/h (168 mph)	High-performance sports cars
Y	300 km/h (186 mph)	Supercars, hypercars

The speed rating is not just about top speed. Higher-rated tyres use stiffer compounds and reinforced sidewalls that improve handling response and braking performance at everyday speeds too. Fitting a lower speed rating than the manufacturer specifies can affect your insurance cover in many countries.

If you are comparing vehicles with different tyre specifications, the EV vs fuel cost calculator can help you understand how tyre choice feeds into overall running costs. Electric vehicles often use specific low-rolling-resistance tyres with higher load ratings to handle the extra battery weight.

Practical Tips for Reading and Choosing Tyre Sizes

The tyre code is moulded into the sidewall and can be hard to read, especially on older or dirty tyres. Look for it on the outer sidewall near the rim. On some vehicles, you will also find the recommended tyre size on a sticker inside the driver's door jamb, on the fuel filler flap, or in the owner's manual. Many cars have two approved sizes listed - one for standard wheels and one for optional larger alloys.

When shopping for replacement tyres, match all four values in the primary code (width, aspect ratio, construction, rim diameter) exactly unless you are deliberately changing size. The load index and speed rating should meet or exceed the manufacturer's original specification. Going lower on either is not safe and may invalidate your insurance.

A common mistake is confusing the aspect ratio with a fixed height. As described above, the aspect ratio is a percentage of the width. A 55 on a 215-wide tyre gives a different sidewall height than a 55 on a 245-wide tyre. This is why changing width alone changes the overall diameter, and why "plus-sizing" (going to a wider tyre on a bigger rim) typically means dropping the aspect ratio to keep the overall diameter constant.

For example, moving from 205/55R16 to 225/45R17 keeps the overall diameter almost identical (632 mm vs 634 mm) while giving a wider contact patch and a lower-profile look. This kind of swap is common when upgrading to aftermarket alloy wheels.

If you are converting between metric and imperial measurements, the km to miles converter can help with distance and speed conversions when comparing tyre specifications across different markets.

The date of manufacture is also stamped on the sidewall, shown as a four-digit DOT code. The last four digits indicate the week and year of production - for example, 2423 means the tyre was made in week 24 of 2023. The UK's National Tyre Distributors Association recommends replacing tyres older than 10 years regardless of tread depth, and inspecting any tyre over 5 years old annually, as the rubber compound degrades with age even if the tyre sees little use.

Finally, tyre pressure matters just as much as tyre size. The correct pressure is set by the vehicle manufacturer (not the tyre maker) and is based on the car's weight distribution and the tyre's load index. Under-inflated tyres wear faster on the edges, increase fuel consumption, and run hotter. Over-inflated tyres wear in the centre and reduce grip. Check pressures monthly when the tyres are cold.

Sources

• ETRTO - European Tyre and Rim Technical Organisation (standards manual)
• U.S. Tire Manufacturers Association - What's on a Sidewall
• UNECE Regulation No. 30 - Pneumatic Tyres for Passenger Cars
• GOV.UK - Tyre Safety and the Law
• National Tyre Distributors Association (UK) - Tyre Age Guidance
• NHTSA - Tires (DOT code and tyre safety)