Tire Size Calculator

Speedometer reading with tires 3% off

Sizes close to 225/45R17

Speed rating symbols

Load index 70 to 126

Reading a sidewall: P225/60R16 97H

How to compare two tire sizes

1. Find the size on the sidewall. Read the size code printed on the tire, for example 225/45R17 94W, or take it from the door-jamb placard or owner’s manual. Flotation sizes read like 33x12.50R15.
2. Enter the current size. Type it in “Current tire size”. Prefixes (P, LT, ST, T), ZR, a C suffix and the load and speed code are all accepted. 225/45R17 94W reads as 24.97 inches tall.
3. Enter the new size. Type the size you are considering in “New tire size”. Leave it empty to see one tire alone. 235/40R18 95Y is 25.40 inches tall.
4. Set the speed you want to check. Enter what the speedometer reads, in mph or km/h. The true speed with the new tire is that reading times the new diameter divided by the old diameter.
5. Check the guidance line. If the diameter differs by more than 3% the tool says so. Then confirm the fitment with the vehicle maker or a tire maker, and check clearance, rim width, load index and speed rating.

Frequently asked questions

How do I read a tire size like 225/45R17?

225 is the section width in millimetres, 45 is the sidewall height as a percentage of that width, R means radial construction and 17 is the rim diameter in inches. The sidewall is 225 × 0.45 = 101.3 mm, and the overall diameter is 2 × the sidewall + 17 × 25.4 mm = 634 mm (24.97 in).

How do I calculate a tire’s diameter, circumference and revolutions per mile?

Diameter = 2 × (width × aspect ratio ÷ 100) + rim diameter × 25.4 mm. Circumference = π × diameter, which is 78.45 in (1,993 mm) for 225/45R17. Revolutions per mile = 63,360 inches ÷ circumference in inches = 807.6, or 501.8 per kilometre.

How do I work out the speedometer error from a different tire size?

True speed = indicated speed × new diameter ÷ old diameter. 225/45R17 to 235/40R18 is a diameter change of +1.72%, so 60 mph on the speedometer is 61.0 mph (98.2 km/h) and 1,000 miles on the odometer is 1,017 mi driven. A taller tire makes the speedometer read low; a shorter tire makes it read high. This assumes the speedometer was accurate with the original tires; many read a little high from the factory.

What is the 3% rule for tire sizes?

It is common guidance, repeated by tire retailers and vehicle makers, that a replacement tire should keep its overall diameter within about 3% of the original to keep the speedometer, odometer and systems that rely on wheel speed (ABS, traction and stability control, some gearing and four-wheel-drive systems) close to what the vehicle was designed for. It is guidance, not a legal limit or an approval: some vehicles allow less, and the vehicle maker’s or a tire maker’s fitment guidance for your car takes precedence. For 225/45R17, 3% is a diameter of 24.22 to 25.72 inches.

What happens if the new tire is more than 3% different?

The tool shows a warning. For example 225/45R17 to 265/50R19 is +17.86%, so 60 mph indicated is 70.7 mph. The larger the change, the larger the speedometer and odometer error, and the more chance of rubbing, of changed gearing and of confusing wheel-speed systems. Check the fitment guidance before you buy.

Does a bigger rim change the speedometer?

Not if the overall diameter stays about the same. Plus-sizing raises the rim and lowers the sidewall so the tire stays the same height: 225/45R17 to 225/40R18 changes the diameter by +0.46%. Only the overall diameter matters for the speedometer; the rim size by itself does not.

How do I read the load index and speed rating, such as 94W?

The number is the load index: 94 is 670 kg (1,477 lb) per tire. The letter is the speed rating: W is 270 km/h (168 mph). The tables on this page list load indexes 70 to 126 and speed symbols L to Y and (Y). A replacement should have at least the load index the vehicle maker specifies, and normally the same or a higher speed rating.

What do P, LT, ST, T, ZR and C mean?

P is a passenger tire (P-metric), LT a light-truck tire, ST a special trailer tire and T a temporary spare; a size with no letter in front is a Euro-metric size. R is radial, D diagonal (bias) and B belted bias; ZR marks a radial tire designed for speeds above 240 km/h (150 mph). A C at the end of the rim size marks a commercial (van) tire. LT and some C tires show two load indexes, such as 121/118: the first is for single fitment, the second for dual (twin) rear wheels.

How do I read a flotation size like 33x12.50R15?

Flotation sizes give the overall diameter first (33 inches), then the section width (12.50 inches) and the rim diameter (15 inches). The sidewall is (33 − 15) ÷ 2 = 9.00 inches. The width is 318 mm and the circumference 103.67 inches, 611.2 revolutions per mile. The diameter is the nominal one printed on the tire; a real tire can measure a little less, especially when loaded.

How accurate are these dimensions?

They are nominal, calculated from the size code. A real tire’s width depends on the wheel it is mounted on (by roughly 5 mm per half-inch of rim width, and manufacturers publish a measured width for each size), the diameter varies between makes and with wear, load and pressure, and the loaded radius is smaller than half the diameter. For an exact figure, use the tire maker’s published specification.

Where does the tire markings information come from?

Tire sidewall markings in the United States are required by the Federal Motor Vehicle Safety Standard 49 CFR 571.139 (FMVSS 139) for passenger-car tires, and NHTSA publishes consumer guidance on reading a sidewall. The load index and speed symbol tables follow the ETRTO and Tire and Rim Association standard tables; see the sources below.

Is anything I type sent anywhere?

No. The calculation runs in your browser and the sizes you enter are not sent or stored. The tool does arithmetic on the size codes only: it cannot know your vehicle, so it cannot tell you a tire will fit or is safe for it.