How to read a tire size
Car, SUV and pickup tires show their size on the sidewall in a standard format. Take P225/45R17 91V as an example:
| Part | Meaning |
|---|---|
| P | Tire type. P is P-metric (passenger), LT is light truck, T is a temporary spare and ST is a special trailer tire. No letter means a Euro-metric size. |
| 225 | Section width in millimeters, sidewall to sidewall. |
| 45 | Aspect ratio: the sidewall height is 45 percent of the width. |
| R | Radial construction, which almost every tire on the road uses. |
| 17 | Rim diameter: the tire fits a 17-inch wheel. |
| 91V | Load index and speed rating. They describe how much weight and speed the tire is built for, not its size, so the calculator ignores them. |
Notice the mixed units: width in millimeters, wheel in inches and the sidewall as a percentage of the width. That is why two sizes that look close can differ more than you expect, and why a calculator helps. The calculator also reads sizes written like 245/40ZR18, 265/75R16LT, 225/45-17 or 225 45 17.
The formulas behind the numbers
Worked example for 225/45R17: the sidewall is 225 × 0.45 = 101.25 mm, or 3.99 inches. The overall diameter is 17 × 25.4 + 2 × 101.25 = 634.3 mm, which is 24.97 inches. The circumference is π × 24.97 = 78.45 inches, and a mile is 63,360 inches, so the tire turns about 807.6 times per mile.
Compare it with 235/40R18: 94 mm of sidewall on an 18-inch wheel gives 645.2 mm, or 25.40 inches. The new size is 1.72 percent taller, so at a speedometer reading of 60 mph you would really be doing 61.0 mph. The axle also sits half the diameter difference higher, here 5.45 mm (0.21 inches).
Flotation sizes like 33x12.50R15
Off-road and light truck tires often use a flotation size, written in inches in a different order. Goodyear lists the parts as overall diameter, section width and rim diameter. In 33x12.50R15, the tire is 33 inches tall and 12.50 inches wide and fits a 15-inch wheel. The sidewall is half of what is left after the wheel: (33 − 15) ÷ 2 = 9 inches. You can mix both systems in the calculator, for example to see how a 33x12.50R15 compares with the LT265/75R16 on a pickup: it is 1.35 inches taller, 4.27 percent.
Why a new size changes your speedometer
Your speedometer and odometer do not measure the road. They count how fast the wheels turn and multiply by the distance one turn should cover with the original tire. A taller tire covers more road per turn, so the speedometer reads low: you drive faster than it shows, and the odometer logs fewer miles than you drove. A shorter tire does the opposite. Michelin points out that changing the overall diameter affects speedometer accuracy and the clearance between the tire and the wheel well, which is exactly what the comparison shows.
The error is a fixed percentage, so it grows with speed. At 1.72 percent it is about half a mile per hour around town and over a mile per hour on the highway.
Common tire sizes compared
Nominal dimensions, worked out from the size alone:
| Size | Sidewall | Diameter | Diameter (mm) | Revs per mile |
|---|---|---|---|---|
| 195/65R15 | 4.99 in | 24.98 in | 635 | 807 |
| 215/55R17 | 4.66 in | 26.31 in | 668 | 767 |
| 225/45R17 | 3.99 in | 24.97 in | 634 | 808 |
| 225/65R17 | 5.76 in | 28.52 in | 724 | 707 |
| 235/65R17 | 6.01 in | 29.03 in | 737 | 695 |
| 245/45R18 | 4.34 in | 26.68 in | 678 | 756 |
| 255/35R19 | 3.51 in | 26.03 in | 661 | 775 |
| 265/70R17 | 7.30 in | 31.61 in | 803 | 638 |
| 275/55R20 | 5.95 in | 31.91 in | 811 | 632 |
| LT265/75R16 | 7.82 in | 31.65 in | 804 | 637 |
| LT285/70R17 | 7.85 in | 32.71 in | 831 | 617 |
| 31x10.50R15 | 8.00 in | 31.00 in | 787 | 651 |
| 33x12.50R15 | 9.00 in | 33.00 in | 838 | 611 |
Before you change tire size
- Start from the placard. Federal rules require the vehicle’s tire placard, normally on the driver’s side door pillar (B-pillar) or the rear edge of the driver’s door, to list the original tire size. The owner’s manual has it too. If the tires on the car now differ from the placard, a previous owner may have changed them.
- Get advice for your vehicle. Michelin recommends following the vehicle manufacturer’s specifications and changing size only with guidance from a trained tire professional. This calculator shows the differences in numbers; it cannot tell you whether a size fits or is safe on your vehicle, so we do not give a percentage limit.
- Keep the load capacity. If your truck came with LT tires, Michelin warns that P-metric tires of the same size leave you short on load capacity.
- Check the rim. The rim number of the tire must match your wheel exactly.
- Look at the width too. Clearance depends on how wide the tire is as well as how tall, so check the width row, not only the diameter.
Why real tires measure a little differently
The numbers here are nominal: they follow from the size on the sidewall. Real tires of the same size from different brands and models vary a bit, and tire makers publish their own measured overall diameter and revolutions per mile. Michelin defines overall diameter as the diameter of the inflated tire without any load, and notes that revolutions per mile can vary with speed, load and inflation pressure. For a tight fit, use the spec sheet of the exact tire you plan to buy.
How to use the tire size calculator
- 1Type the size on your tires now, like 225/45R17, LT265/75R16 or 33x12.50R15. Upper or lower case and spaces do not matter.
- 2Type the size you are thinking about in the second box to compare, or leave it empty to see the dimensions of one tire.
- 3Read the diameter difference and the speedometer check, look at the two tires side by side, then copy the result or the link for your tire shop.
Frequently asked questions
How do I calculate the diameter of a tire from its size?
Multiply the width by the aspect ratio to get the sidewall height, double it (a tire has a sidewall above and below the wheel) and add the rim diameter. For 225/45R17: 225 mm × 0.45 = 101.25 mm of sidewall, which is 3.99 inches. 17 + 2 × 3.99 = 24.97 inches, or 634.3 mm.
What does 225/45R17 mean?
225 is the section width in millimeters, 45 is the aspect ratio (the sidewall height is 45 percent of the width), R means radial construction and 17 is the wheel diameter in inches. A P in front marks a P-metric passenger tire, LT a light truck tire, and no letter a Euro-metric size.
How much does a bigger tire change my speedometer?
By the same percentage as the diameter. Your speedometer counts wheel turns and assumes the original tire size, so a tire 2 percent taller covers 2 percent more road per turn. When the speedometer shows 60 mph you are then really doing about 61.2 mph. The odometer undercounts your miles by the same amount.
What does 33x12.50R15 mean?
It is a flotation size, written in inches: 33 inches overall diameter, 12.50 inches section width, R for radial and a 15-inch rim. Because the diameter is part of the size, the sidewall is simply (33 − 15) ÷ 2 = 9 inches. The calculator compares flotation sizes with metric sizes like LT265/75R16.
Can I put a different tire size on my car?
Check before you do. The size your vehicle was built for is on the tire placard on the driver’s side door pillar or door edge and in the owner’s manual. Michelin advises following the vehicle manufacturer’s specifications and changing size only with guidance from a trained tire professional, because a different overall diameter changes speedometer accuracy and clearance.
Why does a tire maker list a different diameter than this calculator?
This calculator gives the nominal size that follows from the numbers on the sidewall. A real tire is measured on a specific wheel and pressure, and its revolutions per mile can vary with speed, load and inflation pressure. For exact clearance, use the spec sheet for the exact tire you plan to buy.
What do P, LT, T and ST mean in front of a tire size?
P stands for P-metric passenger tires, LT for light truck tires, T for a temporary spare and ST for special trailer tires. A size with no letter in front is a Euro-metric size. The letters do not change the math for the dimensions, so the calculator reads all of them.