Compare two sizes with our tyre size calculator
See how two tyre sizes compare in diameter, sidewall, circumference and revolutions per mile, and what a change does to your speedometer. Only fit sizes approved for your car.
How do you compare two tyre sizes?
Compare their overall diameter. Each sidewall is the width multiplied by the profile percentage, and the diameter is two sidewalls plus the rim size in millimetres. A 205/55 R16 is 631.9 mm across and a 225/45 R17 is 634.3 mm, just 0.4% bigger.
Check before you change size. Only fit a tyre size your car's manufacturer approves for your model, with a load index and speed rating at least as high as the car requires. The approved sizes are in the handbook and often on the door pillar placard. Changing wheels or tyres away from the original specification can count as a modification, so speak to your insurer first.
| Measurement | 205/55 R16 | 225/45 R17 | Difference |
|---|---|---|---|
| Overall diameter | 631.9 mm | 634.3 mm | +2.4 mm (+0.4%) |
| Sidewall height | 112.8 mm | 101.3 mm | -11.5 mm |
| Circumference | 1,985.2 mm | 1,992.7 mm | +7.5 mm |
| Revolutions per mile | 811 | 808 | -3 |
| Real speed when the speedometer shows 30 mph | 30.0 mph | 30.1 mph | +0.1 mph |
| Real speed when the speedometer shows 50 mph | 50.0 mph | 50.2 mph | +0.2 mph |
| Real speed when the speedometer shows 70 mph | 70.0 mph | 70.3 mph | +0.3 mph |
225/45 R17 is 0.4% larger in overall diameter than 205/55 R16. When the speedometer shows 70 mph you would be doing about 70.3 mph, so it would read lower than your real speed. Only fit it if it is an approved size for your car.
How do you read a tyre size like 205/55 R16 91V?
Read it left to right. The first three numbers set the tyre's physical size, and the last number and letter set how much weight it can carry and how fast it can go.
| 205 | Tyre width in millimetres. |
|---|---|
| 55 | Profile, or aspect ratio: the sidewall height as a percentage of the width. Here 55% of 205 mm, which is 112.75 mm. |
| R | Radial construction, used on almost all modern car tyres. |
| 16 | Rim diameter in inches: the size of wheel the tyre fits. |
| 91 | Load index: a number code for the maximum load each tyre can carry, looked up in a standard load index table. |
| V | Speed rating: a letter code for the maximum speed the tyre is built for. |
The calculator only needs width, profile and rim, because those set the dimensions. The load index and speed rating do not change the size, but they matter just as much when you choose a replacement. Our guides on how to read tyre size and load index and speed ratings cover the rest of the sidewall.
How are the figures worked out?
Every figure comes from the size markings, so you can check any of them with a calculator.
- Sidewall height = width × profile ÷ 100. For 205/55, that is 205 × 0.55 = 112.75 mm.
- Overall diameter = (rim in inches × 25.4) + (2 × sidewall). For 205/55 R16, that is 406.4 + 225.5 = 631.9 mm.
- Circumference = diameter × π. A 631.9 mm tyre goes round once every 1,985.2 mm.
- Revolutions per mile = 1,609,344 ÷ circumference in mm, because a mile is 1,609,344 mm. A 205/55 R16 turns about 811 times a mile.
- Real speed = speedometer reading × new diameter ÷ original diameter.
These are nominal figures. Real tyres differ slightly between brands and models, and a tyre gets smaller as the tread wears, so treat very small differences as approximate.
What does a bigger tyre do to your speedometer?
It makes the speedometer read lower than your real speed. A smaller tyre does the opposite and makes it read higher.
Your speedometer is set up for the tyre size the car was built with. It works out speed from how quickly the wheels turn, so if the new tyre is taller, each turn covers more ground and you travel faster than the dial shows. The mileage the car records changes in the same way.
As an example, swap a 205/55 R16 for a 225/50 R17. The new tyre is 656.8 mm across, 3.9% taller. When the speedometer reads 30 mph you are doing about 31.2 mph, at 50 mph about 52.0 mph, and at 70 mph about 72.8 mph. Revolutions per mile drop from about 811 to about 780.
Compare that with 225/45 R17, which keeps the diameter within 2.4 mm of the 205/55 R16 on a 17-inch wheel. A lower profile on a bigger rim is how larger wheels keep the overall size close to standard. Diameter also affects more than the speedometer: the tyre has to clear the wheel arch, suspension and brakes at full steering lock and with the car loaded, which is why the manufacturer's approved list matters.
Can you fit a different tyre size to your car?
Only a size your car's manufacturer approves. A calculator can tell you two sizes are close in diameter. It cannot tell you whether a tyre suits your car.
- Approved sizes. The handbook and the door pillar placard list the sizes the car was sold with, and many models were offered with more than one wheel size. If a size is not listed, ask the manufacturer or a dealer before you fit it.
- Load index and speed rating. A replacement needs a load index and speed rating at least as high as the car requires. See our load and speed rating guide.
- Matching tyres across an axle. Tyres on the same axle should be the same size and type, which is why tyres are usually replaced in pairs.
- Insurance. The AA advises talking to your insurer before you modify a car, and names alloy wheels as the most popular modification. Ask before you change wheel or tyre size.
- Pressures. A different size can need a different pressure, so use the handbook figure for the size fitted. Our tyre pressure converter turns bar into PSI.
Not sure what is approved? Send a photo of the tyre sidewall and the door pillar placard on WhatsApp. We source budget, mid-range and premium tyres to your car's size and fit them where the car is parked.
Tyre sizes, answered.
Sources: the MOT inspection manual on tyre size, load capacity and speed rating; the AA on car modifications and insurance. Diameter, circumference and speed figures are calculated from the size markings.
What does 205/55 R16 mean?
How do I work out a tyre's overall diameter?
Will a bigger tyre make my speedometer wrong?
Can I fit a different tyre size to my car?
Are 225/45 R17 and 205/55 R16 the same size?
Do I need to tell my insurer if I change tyre size?
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