Tire Size Calculator
Type a sidewall code and read every dimension it defines: section width in millimetres and inches, sidewall height, rim diameter, overall diameter, circumference and revolutions per mile. Put a second size beside it and the page adds the diameter difference in inches and as a percentage, what the speedometer reads when you are truly doing 60 mph, and the miles the odometer records against 100 actually driven. The code is a definition rather than a measurement, so the arithmetic is exact, and there is no catalogue behind any of it.
Current
- Section width
- 225 mm (8.86 in)
- Sidewall (45%)
- 3.99 in (101.3 mm)
- Rim diameter
- 17 in (431.8 mm)
- Overall diameter
- 24.97 in (634.3 mm)
- Circumference
- 78.45 in (1992.7 mm)
- Revolutions per mile
- 808
New
- Section width
- 235 mm (9.25 in)
- Sidewall (40%)
- 3.70 in (94.0 mm)
- Rim diameter
- 18 in (457.2 mm)
- Overall diameter
- 25.40 in (645.2 mm)
- Circumference
- 79.80 in (2027.0 mm)
- Revolutions per mile
- 794
Diameter
Two codes side by side, with the diameter difference and what it does to the speedometer.
What the change does
The speedometer and the odometer are out by the same percentage as the diameter, at every speed and in any units: a taller tire turns fewer times per mile, so the needle and the mileage both read low.
Geometry, not fitment. The page renders no will-it-fit verdict, no clearance claim, no load or speed rating suggestion and no inflation pressure, and it holds no vehicle size table. Exact arithmetic on the sidewall code, which is a definition rather than an estimate. It cannot know your wheel well's clearance, whether a load or speed rating suits your vehicle, or the true mounted diameter, which changes with pressure, load and wear.
Common questions
- How do I read a tire size such as 225/45R17?
- 225 is the section width in millimetres, measured across the widest part of the inflated tire. 45 is the aspect ratio: the sidewall height as a percentage of that width, so 225 times 45% is 101.25 mm of sidewall. R is radial construction (D is diagonal bias, B is belted bias). 17 is the rim diameter in inches, which stays in inches even though the width is metric. The overall diameter is the rim plus two sidewalls: 17 plus 2 times 101.25 divided by 25.4, which is 24.97 inches. The page shows each of those steps as it reads your code.
- How much diameter difference is acceptable?
- That depends on your car, and this page will not answer it with a number. Clearance in the wheel well, suspension travel, how the brake and traction systems were calibrated and the manufacturer's own tolerance all decide it, and none of them are visible from a sidewall code. A figure printed here would read as permission from a page that cannot see your vehicle. What the page gives you is the exact difference, in inches and as a percentage, to take to your vehicle's documentation or to a fitter.
- How does a size change affect the speedometer?
- A speedometer counts wheel revolutions and multiplies by the circumference it was calibrated for, which is the size the car left the factory on. Fit a taller tire and the wheel turns fewer times for the same distance, so the needle reads low; fit a shorter one and it reads high. The reading at a true 60 mph is 60 times the current diameter divided by the new diameter, so 225/45R17 to 235/40R18 shows 58.99 mph at a true 60. The error is the same percentage at every speed and in any units, and the odometer is out by the same percentage over any distance.
- Can this tell me whether a size will fit my car?
- No, and it holds nothing that pretends to. There is no year, make and model lookup here and no original-equipment size table, because a fitment database is scraped from somewhere else and revised every model year, and a stale one is worse than none. Whether a tire fits is also physical: clearance at full lock and full compression is something you check on the car. The page reports geometry, exactly, and stops there.
- What is the difference between section width and tread width?
- Section width is the widest point of the inflated tire, sidewall bulges included, measured on the maker's reference rim. Tread width is only the part of the rubber that meets the road, and it is always narrower. The code carries the section width, so that is what this page reports; tread width varies by pattern and is not derivable from the code. Fitting the same size on a wider rim also stretches the section width slightly, which is one reason a mounted tire rarely measures exactly what the code says.
- What are revolutions per mile?
- How many times the wheel turns to cover a mile: 63,360 inches in a mile divided by the tire's circumference. It is the figure a speedometer, an odometer and a wheel-speed sensor are all effectively counting, which is why two sizes with different revolutions per mile disagree about your speed. A 225/45R17 turns about 808 times a mile and a 235/40R18 about 794.
- Is anything uploaded?
- No. Reading the code and working out the dimensions both happen in this page, in your browser, and the tool makes no network request while it calculates. The two sizes you last typed and your choice of inches or millimetres are kept in your own browser's local storage so the page opens where you left it, and nothing else is stored.
Exact arithmetic on the sidewall code, which is a definition rather than an estimate. It cannot know your wheel well's clearance, whether a load or speed rating suits your vehicle, or the true mounted diameter, which changes with pressure, load and wear.