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Tire Size Calculator
Compare two tire sizes such as 205/55R16 and 225/45R17: overall diameter, width, sidewall height, revolutions per mile, and how far off the speedometer will read.
Tire Size Calculator: In a size like 225/45R17, 225 is the width in millimetres, 45 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. The calculator works out the overall diameter (wheel plus two sidewalls), the circumference, and the revolutions per mile or kilometre, and compares the two sizes. Flotation sizes such as 31x10.50R15 are read too. Runs 100% locally in your browser with zero server file uploads.
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Diameter +0.38%
At 60 on the speedometer you are doing 60.2| 205/55R16 | 225/45R17 | |
|---|---|---|
| Overall diameter | 24.88 in | 24.97 in |
| Section width | 8.07 in | 8.86 in |
| Sidewall height | 4.44 in | 3.99 in |
| Rim diameter | 16 in | 17 in |
| Circumference | 78.16 in | 78.45 in |
| Revolutions per mile | 811 | 808 |
Sizes are nominal, from the numbers on the sidewall; real tires differ a little by brand, wear, and pressure. The sizes approved for your car are on the placard inside the driver's door or in the owner's manual.
Plus sizing
Fitting a larger wheel with a lower-profile tire, such as going from 205/55R16 to 225/45R17, is called plus sizing. Done well, the overall diameter stays almost the same, so the speedometer and gearing are unaffected, while the shorter sidewall sharpens steering at the cost of a firmer ride and more exposed wheels.
Wider tires add grip in the dry but can follow ruts and aquaplane sooner in heavy rain, and they may cost a little fuel. To see how a different fuel use affects a trip, use the fuel cost calculator.
Reading the sidewall
Besides the size, the sidewall carries the load index and speed rating, the DOT code with the week and year of manufacture (2523 means week 25 of 2023), and the maximum pressure, which is not the pressure to run at. The right pressure for your car is on the door-frame placard.
How to use it
- Enter your current tire size from the sidewall, such as 205/55R16.
- Enter the size you are considering.
- Read the difference in diameter and the true speed at a given speedometer reading.
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Frequently asked questions
What do the numbers on a tire mean?
In P225/45ZR17 91W, P means passenger car, 225 is the width in millimetres, 45 the aspect ratio, Z a speed-rating mark, R radial construction, 17 the wheel diameter in inches, 91 the load index, and W the speed rating. Load index and speed rating should be at least those of the original tires.
How much can the diameter change?
A common rule of thumb is to stay within about 3% of the original overall diameter. Beyond that the speedometer and odometer read noticeably wrong, ABS and stability systems may be affected, and the tire may rub. Your vehicle's manual or the placard in the door frame lists the approved sizes.
Why does a bigger tire make my speedometer read low?
The speedometer counts wheel turns. A taller tire travels further each turn, so at an indicated 60 mph you are going a little faster than 60. A smaller tire makes it read high.
Can I fit a wider tire on my wheel?
Each tire size has a range of approved rim widths, given by the tire maker. Check that range, and the clearance to the suspension and wheel arch, before changing width.
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