Compare two tyre sizes side by side — diameter, sidewall, revolutions per mile — and see the speedometer error a change would cause.
A size like 225/45R17 is width 225, aspect 45, rim 17.
A marking like 225/45R17 encodes three things in three different unit systems, which is why it confuses people.
225 is the section width in millimetres. 45 is the aspect ratio — the sidewall height as a percentage of the width, not an absolute measurement. R means radial construction. 17 is the wheel diameter in inches.
So the sidewall here is 225 × 0.45 = 101.25 mm, and the overall diameter is the rim plus two sidewalls: 17 × 25.4 + 2 × 101.25 = 634.3 mm, or 24.97 inches.
Because it is relative, changing the width changes the sidewall height even if the aspect number stays the same. Going from 225/45R17 to 245/45R17 does not just make the tyre wider — it makes it taller too, because 45% of 245 is more than 45% of 225. To keep the diameter roughly constant while going wider, you have to reduce the aspect ratio.
That is the logic behind "plus sizing": go up an inch in rim and down in aspect ratio, keeping overall diameter close to standard. 225/45R17 to 245/40R18 is a textbook example — it comes out just under 3% larger.
Replacement tyres are normally kept within 3% of the original overall diameter. Beyond that, several things go wrong at once.
Speedometer and odometer error. A larger tyre covers more ground per revolution, so the car is travelling faster than the speedometer indicates. On a 3% oversize, an indicated 60 is really about 61.8. Your odometer under-records too, which quietly affects service intervals, warranty mileage and resale.
ABS and stability control. These systems infer wheel speed and slip from rotation, calibrated to the factory rolling circumference. A significant change degrades their calculations.
Transmission behaviour. Automatic shift points are mapped against vehicle speed, so a large diameter change effectively alters the final drive ratio and shifts the whole map.
Clearance. A taller or wider tyre can foul suspension components or the inner wing at full lock or full compression — often only under conditions you will not discover in a car park.
A lower profile tyre — smaller aspect ratio — has a shorter, stiffer sidewall. That means sharper steering response and less roll, which is why performance cars use them. It also means the sidewall absorbs far less, so ride quality suffers and the wheel is much more vulnerable to pothole damage. Low-profile tyres on large rims are one of the most common causes of buckled alloys.
Wider tyres increase the contact patch, which generally improves dry grip and cornering. They also raise rolling resistance slightly, are more prone to aquaplaning at speed because there is more water to displace, and cost more.
Beyond the size you will find a load index and speed rating, such as 91V. Never fit a tyre with a lower load index than the manufacturer specifies. There is also a four-digit DOT date code giving the week and year of manufacture — 3224 means the 32nd week of 2024. Rubber degrades whether or not it is used, and tyres over about six years old are worth inspecting closely regardless of tread depth.
The treadwear, traction and temperature grades of the UTQG rating are also moulded in, though treadwear numbers are only comparable within a single manufacturer's range.
In 225/45R17: 225 is the section width in millimetres, 45 is the aspect ratio — sidewall height as a percentage of the width, not an absolute figure — R indicates radial construction, and 17 is the wheel diameter in inches. Three different unit systems in one marking, which is why it looks arbitrary. The sidewall here is 225 × 0.45 = 101.25 mm.
Proportionally to the diameter change. A tyre 3% larger means you are actually travelling about 3% faster than indicated — an indicated 60 is really 61.8. The odometer under-records by the same proportion, which affects service intervals and recorded mileage. Smaller tyres do the reverse, making the speedometer over-read. Most manufacturers keep replacements within 3% for this reason.
Often, within limits. Each rim width supports a range of tyre widths, typically about 20 mm either side of the nominal fit. Going wider without changing the aspect ratio also makes the tyre taller, since aspect is a percentage of width — so you usually need to drop the aspect ratio to keep the diameter close. Check clearance at full steering lock and full suspension compression, not just in a straight line.
Increasing rim diameter by an inch while reducing the tyre aspect ratio, so the overall diameter stays close to standard. Going from 225/45R17 to 245/40R18 is a typical example — it comes out under 3% larger. The result is more responsive steering from the stiffer, shorter sidewall, at the cost of ride comfort and greater vulnerability to pothole damage. It is the standard way to fit larger wheels without upsetting the speedometer.
The four-digit DOT code on the sidewall gives the week and year of manufacture — 3224 means week 32 of 2024. Rubber hardens and degrades with age regardless of use, so many manufacturers recommend replacement at six years and most set a firm limit at ten, whatever the tread depth. This matters especially for spare tyres and low-mileage vehicles, where a tyre can look almost new and still be past its safe life.
Related: Gas Mileage Calculator · Speed Calculator · Horsepower Calculator · Circle Calculator Other vehicle tools: the Horsepower Calculator and the Gas Mileage Calculator.