Tire Size Calculator

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.

Can it read inch sizes like 33x12.50R15, or show a metric size in inches?

Both. An inch size states the overall diameter first, then the section width, then the rim, so 33x12.50R15 is a 33 inch tire, 12.50 inches across, on a 15 inch rim, which leaves 9 inches of sidewall on each side. LT before or after the code is read and dropped. Every metric size is also written out in inch terms to a tenth of an inch, so 265/70R17 shows as 31.6x10.4R17, and an inch size is written in metric terms as a width in millimetres and an aspect ratio. Put one of each into the compare fields to see a metric and an inch size side by side.

How do gear ratio and tire size change engine rpm?

Engine rpm is road speed times the axle ratio times the transmission gear ratio times 63,360, divided by 60 times the tire circumference in inches. A taller tire has a longer circumference, so the engine turns slower at the same speed. With a 3.73 axle in a 1.00 top gear at 60 mph, 265/70R17 turns the engine at 2,380 rpm and 315/70R17 at 2,189. The compare view works this out for both sizes, with road speed per 1,000 rpm and the axle ratio that would give the new size the same engine speed as now, which for that pair is 4.06.

Does it work for motorcycle, ATV, golf cart and mower tires?

For the codes it reads, yes. Metric motorcycle sizes such as 180/55ZR17 and 120/70-17 read the same way as car sizes, as long as the width is 100 mm or more and the aspect ratio is 95 or less, so narrow off-road fronts such as 90/90-21 are outside it. The construction mark does not enter the arithmetic, so a dash code such as 120/70-17 is shown back as 120/70R17 with the same figures; the R there is the plain spelling, not a claim that the tire is radial. Inch sizes on ATV, golf cart and mower tires, such as 25x8-12, 18x8.50-8 and 15x6.00-6, read as inch codes. Bicycle sizes such as 700x25c or 29x2.2 use a different system and are not read here.

How do I find the size of a spare tire cover?

Read the code off the spare itself, on its sidewall, and type it in: covers are described by the tire overall diameter and its width, and the page gives both in inches and millimetres. 265/70R17 is 31.61 inches across and 10.43 inches wide. Cover makers each publish their own size chart, so match these two figures against the chart of the cover you are looking at.

Is there an app to download?

There is nothing to install. The calculator is this page, and it works in any current browser on a phone, a tablet, a Chromebook or a computer, including Safari on an iPhone and Chrome on Android. The sizes you last typed stay in your browser, so it opens where you left it.

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.