How to use the tire size calculator
Type the size of the tires on your vehicle in the first box and the size you are thinking about in the second. A size looks like 265/70R17 and is printed on the sidewall. Add a load index and speed symbol after it, like 265/70R17 115T, if you want those compared. Flotation sizes like 33x12.50R17 work too. The picture draws both tires to scale, and the table shows how tall, wide and fast each one turns. The steppers under each box change the width, the profile or the wheel one step at a time. Nothing you type leaves your device unless you make a share link.
How a tire size is read
In 265/70R17, the 265 is the section width in millimeters, the 70 is the height of the sidewall as a percent of that width, the R means radial, and the 17 is the wheel diameter in inches. A letter in front, such as P for passenger car or LT for light truck, names the class. In a flotation size like 33x12.50R17, the 33 is the overall height of the tire in inches, the 12.50 is its width in inches, and 17 is the wheel.
The formulas
overall height = wheel × 25.4 + 2 × sidewall (all in mm)
circumference = π × overall height
revolutions per mile = 63,360 in ÷ circumference
The wheel is in inches, so it is multiplied by 25.4 to get millimeters. The sidewall counts twice because the tire has one above the wheel and one below. A mile is exactly 63,360 inches, so dividing by the circumference gives the turns in a mile.
A worked example
Say the stock tire is a 265/70R17 and you are looking at a 285/70R17. The stock sidewall is 265 × 70 ÷ 100 = 185.5 mm. The wheel is 17 × 25.4 = 431.8 mm. The overall height is 431.8 + 2 × 185.5 = 802.8 mm, or 31.61 inches. The new sidewall is 199.5 mm, so the new tire stands 830.8 mm, or 32.71 inches. That is 28 mm (1.10 inches) taller, a difference of 3.49 percent. The stock tire turns 638.1 times a mile and the new one 616.6 times. The page opens on this example.
What the speedometer and odometer will say
The speedometer and odometer count wheel turns. A taller tire covers more road with each turn, so both read low. Divide the new height by the stock height: 830.8 ÷ 802.8 = 1.0349. If the dial read true on the stock tire, 60 mph on the dial is really 62.1 mph, and 100 miles on the odometer is really 103.5 miles. At the same road speed and gear, the engine turns about 3.4 percent slower, and the axle rises half the difference in height, about 14 mm (0.55 inch). Gearing, shift points and stability control were set for the stock height, so how much change is acceptable is the vehicle maker's call.
How much change is too much?
There is no single number. Tire sellers quote a rule that the overall height should stay within 3 percent of stock, and others say 5 percent. We found neither written by a standards body, so treat them as rules of thumb. The limit that counts is your vehicle maker's. The page shows the difference and never says a size is safe for your vehicle.
The same height on a bigger wheel
When the wheel goes up, the sidewall gets shorter to keep the overall height. The "Same height on other wheels" list tries each common width near yours on wheels from one size down to four up, and shows the profile that lands closest. For a 265/70R17 it finds 265/65R18, which is 0.14 percent shorter, and 265/60R19, which is 0.27 percent shorter. A new wheel has its own width, offset and clearance, so use the list as a starting point for a talk with a tire shop.
Load index and speed symbol
The load index is the number after the size, and each number is a load per tire. Index 97 is 1,609 pounds (730 kg), and index 115 is 2,679 pounds (1,215 kg), from Tire Rack's table. The speed symbol is the letter after it: T is up to 118 mph (190 km/h), H is 130 mph (210 km/h), and V is 149 mph (240 km/h). Compare the new tire's numbers with the placard on the driver's door jamb and your owner's manual. The page flags a lower load index, but the placard and the manual decide.
How old is the tire?
The last four digits of the DOT number are the date. The code 3524 means week 35 of 2024. Michelin recommends a professional inspection at least once a year once a tire is five years old, and replacement ten years after it was made, even with tread left. Some vehicle makers say six years. The page reads the code. It cannot see the rubber.
Where the numbers come from
The size formulas are the ones Tire Rack and Wikipedia's tire code article use. Speed symbols are Wikipedia's km/h figures, with miles per hour converted and rounded. The load table is Tire Rack's. An inch is exactly 25.4 mm. Sizes are nominal: the tire you buy can differ a little by maker and by wheel width.
Common mistakes
- Comparing wheel sizes instead of overall height. A bigger wheel with a shorter sidewall can match the old height.
- Forgetting the width. A wider tire can rub even when the height matches.
- Ignoring the load index. A lower number carries less, whatever the size.
Questions
How do I find my tire size?
Read it off the sidewall. The placard, usually on the driver's door jamb, and the owner's manual list the size the vehicle came with.
Is a taller tire okay for my vehicle?
That depends on the vehicle: gearing, stability control, clearance and load rating all matter. The page shows the numbers. Ask the vehicle maker or a tire shop whether a size suits yours.
Can I use metric?
Yes. Press Metric to see heights and widths in millimeters, speeds in km/h and turns per kilometer. Tire sizes stay as printed.
Can I share my result?
Yes. Share makes a link with a picture for Facebook, X and Pinterest, a link you can copy, and a short video for TikTok made on your device. The link keeps only the tire sizes, for 90 days.