Tire Size Calculator
Type the three numbers from your current tire and the size you are thinking of fitting. The calculator shows how much taller or shorter the new tire is and what that does to your speedometer.
- Metric tire codes
- Speedometer check
- 3% rule built in
Diameters are calculated from the size code. Real tires vary by a few millimetres between brands and tread depths.
A tire code such as 225/45 R17 holds everything you need. 225 is the tread width in millimetres, 45 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. From those three numbers you can work out the full diameter of the tire, which is what decides gearing, ground clearance and the speed your dashboard shows.
Common tire sizes and their diameters
These are the sizes fitted to most hatchbacks, saloons and small SUVs. Filter by rim size or width, then tap a row to read it in full.
| Size | Sidewall (mm) | Diameter (mm) | Diameter (in) | Revs per km |
|---|---|---|---|---|
| 175/65 R14 | 113.8 | 583.2 | 22.96 | 546 |
| 185/65 R15 | 120.3 | 621.6 | 24.47 | 512 |
| 195/65 R15 | 126.8 | 634.6 | 24.98 | 502 |
| 205/55 R16 | 112.8 | 631 | 24.84 | 504 |
| 215/60 R16 | 129 | 664.4 | 26.16 | 479 |
| 225/45 R17 | 101.3 | 634.4 | 24.98 | 502 |
| 225/50 R17 | 112.5 | 656.8 | 25.86 | 485 |
| 235/45 R18 | 105.8 | 668.8 | 26.33 | 476 |
| 225/40 R18 | 90 | 637.2 | 25.09 | 500 |
| 245/40 R19 | 98 | 678.6 | 26.72 | 469 |
| 255/35 R19 | 89.3 | 661.2 | 26.03 | 481 |
| 235/65 R17 | 152.8 | 737.4 | 29.03 | 432 |
| 265/70 R17 | 185.5 | 802.8 | 31.61 | 397 |
| 275/40 R20 | 110 | 728 | 28.66 | 437 |
| 285/45 R22 | 128.3 | 815.4 | 32.1 | 390 |
Sizes on the same rim with a similar diameter are the usual swap options. A 205/55 R16 and a 225/45 R17 are less than 1% apart, which is why so many cars list both.
See how the aspect ratio changes a tire
Width and rim stay at 225 and 17 inches here. Drag the slider to change only the aspect ratio and watch the sidewall and diameter move.
Every 5 points of aspect ratio on a 225 tire adds or removes about 11 mm of sidewall on each side. That is why going up one rim size usually means going down one or two aspect steps to keep the same rolling diameter.
How to read the size on your tire
The code is moulded into the sidewall in raised letters. Step through each part of a code like 225/45 R17 94W.
Width
The first number, 225, is the tread width in millimetres measured from sidewall to sidewall.
Only the first three numbers matter for size. The load index and speed rating still need to match or beat the original tire, because they decide how much weight and speed the tire is approved for.
Taller, shorter or the same height
Each direction has its own trade-off. Pick an option to see what changes on the road.
A plus-size swap such as 205/55 R16 to 225/45 R17 keeps the diameter within 1%. Gearing, speedometer and clearance stay as designed.
A bigger diameter raises the car slightly and makes the engine turn fewer revs per mile. Your speedometer will read lower than your real speed.
A smaller diameter gives quicker acceleration but higher revs at cruising speed. The speedometer reads higher than your true speed.
Most manufacturers and tire fitters work to a 3% limit either way. Beyond that the car's stability control and ABS can misread wheel speed, and the speedometer may show less than your real speed, which is the risky direction.
Before you fit a different tire size
Diameter is only one of the checks. Tick these off before you order.
Tick each item as you go.
If you are unsure about any of them, the tire placard on the driver's door pillar or in the fuel flap lists every size the maker approved for your car.
What the calculator tells you
Every result comes straight from the tire code. Each tile explains one result.
Rim diameter plus two sidewalls. It decides gearing and clearance.
Taller sidewalls ride softer and protect the rim from potholes.
Your true speed when the dial shows 60, after the change.
How many times the wheel turns each kilometre. Fewer revs means a taller tire.
A quick check against the limit most fitters work to.
None of these figures need you to measure the tire. The size code is a precise description, so the maths is the same one tire fitters and wheel retailers use.
Frequently Asked Questions
No question matches that search.
How do I calculate tire diameter?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall height in millimetres. Double it and add the rim diameter converted to millimetres (inches × 25.4). For 225/45 R17 that is 101.25 × 2 + 431.8 = 634.3 mm.
How much bigger can I go on tire size?
Most tire fitters and car makers work to a limit of 3% larger or smaller in overall diameter. Inside that range the speedometer, ABS and gearing stay close to how the car was designed.
Will a bigger tire make my speedometer wrong?
Yes. A taller tire covers more ground per turn, so the car travels faster than the speedometer shows. A 3% taller tire means about 61.8 mph when the dial says 60.
Can I put a 225 tire where a 205 was?
Often yes, if the rim width is in the approved range for a 225 tire and the diameter stays within 3%. Check the rim width stamped inside the wheel and the clearance in the wheel arch.
What does the R in a tire size mean?
R stands for radial construction, which almost every car tire made today uses. You may also see ZR on older high-speed tires, which marks a rating above 240 km/h.
Is 225/45 R17 the same as 205/55 R16?
Not the same, but close. Their diameters are 634 mm and 631 mm, a difference of about 0.5%, so they are common swaps on the same car with different wheels.
Do wider tires use more fuel?
Slightly. A wider contact patch adds rolling resistance and the tire is usually heavier. The difference is small for one size step but can reach a few percent for large jumps.
How do I convert a metric tire size to inches?
Divide the overall diameter in millimetres by 25.4. The calculator does this for you and shows both units.
Pick a size you can trust
Start with the size on your door placard, try the new size in the calculator and keep the diameter within 3%. Then match the load index and speed rating, and you can fit the new tire knowing the car will drive and measure speed the way it should.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.