Tire tool
Tire size calculator and comparison
Type a tire size to see its sidewall height, overall diameter and revolutions per mile. When a used tire in your size is not in stock and the shop offers a near size, add it as the second size: the comparison shows how much taller or shorter it is, what your speedometer will read, and whether the difference is inside the 3 percent guideline a tire maker gives.
What the numbers mean
NHTSA explained the size designation when it set up its tire consumer information program, using 185/65R14 as the example[Tire Fuel Efficiency Consumer Information Program (final rule)]:
“in passenger car tire sizes (e.g., 185/65R14), the first three numbers indicate the nominal width of the tire, i.e., the width in millimeters from sidewall edge to sidewall edge (185).” [Tire Fuel Efficiency Consumer Information Program (final rule)]
“The second two numbers in the size designation indicate the ratio of tire height to tire width, or the aspect ratio (65).” [Tire Fuel Efficiency Consumer Information Program (final rule)]
“The “R” indicates that this particular tire is a radial tire, as opposed to bias ply construction, which is indicated by a “D” in the size specification, or bias-belted construction, which is indicated by a “B” in the size specification.” [Tire Fuel Efficiency Consumer Information Program (final rule)]
“The last two numbers in the size designation indicate the rim diameter code (14), or the wheel or rim diameter in inches. A change in any of these three numbers indicates a different size specification for a replacement tire.” [Tire Fuel Efficiency Consumer Information Program (final rule)]
A size may start with a letter: LT marks a light truck tire, which federal Standard No. 139 defines as a tire its maker designates “primarily intended for use on lightweight trucks or multipurpose passenger vehicles” [49 CFR 571.139]. Bridgestone adds two more:
“Similarly, "T" stands for "temporary" and is for your spare tire. If you see "ST," that means "special trailer."” [Bridgestone tire size]
Two or three digits and a letter after the size (for example 95H) are the load index and the speed rating. Truck sizes written in inches, such as 33x12.50R17, are a different system: the size converter turns them into metric.
How the calculator works
Sidewall height is the width times the aspect ratio: a 215/65 tire has a sidewall of 215 × 0.65 = 139.75 mm, or 139.75 / 25.4 = 5.50 inches. The overall diameter is the rim plus two sidewalls: 16 + 2 × 5.50 = 27.00 inches. Circumference is the diameter times π (84.8 inches), and revolutions per mile are the 63,360 inches in a mile divided by the circumference (747). These are nominal figures from the size alone: a real tire's measured size varies by model, inflation and wear.
Comparing two sizes: the speedometer and the 3 percent guideline
A speedometer and an odometer count wheel turns. A tire that is larger in diameter covers more road per turn, so the true speed and distance are larger than the readings by the same share; a smaller tire reads high. BFGoodrich gives a guideline for how far a size may move:
“As a general guideline, keep overall diameter within about 3 percent of original. Beyond that, speedometer and odometer error becomes significant, ABS and stability control calibration is affected, and on some vehicles transmission shift points are calculated from wheel speed.” [BFGoodrich]
The calculator reports every comparison against that 3 percent line. Two worked examples:
| 215/65R16 → 225/65R16 | 245/75R16 → 265/75R16 | |
|---|---|---|
| Overall diameter | 27.00 → 27.52 in | 30.47 → 31.65 in |
| Difference | +1.90% | +3.88% |
| Within 3%? | Yes | No |
| Speedometer reads 30 mph | true speed 30.6 mph | true speed 31.2 mph |
| Speedometer reads 45 mph | true speed 45.9 mph | true speed 46.7 mph |
| Speedometer reads 60 mph | true speed 61.1 mph | true speed 62.3 mph |
| Speedometer reads 70 mph | true speed 71.3 mph | true speed 72.7 mph |
| Odometer counts 1,000 miles | 1,019 driven | 1,039 driven |
| Axle height change | +0.26 in | +0.59 in |
| Revolutions per mile | 747 → 733 | 662 → 637 |
The percentages are worked out from the unrounded diameters. The axle height change is half the diameter difference: the axle sits at the center of the tire. A size inside the guideline still has to meet your vehicle's needs in other ways: the same or a higher load index and speed rating, and room in the wheel well. NHTSA's note on the size designation stands: a change in any of the three numbers is a different size specification [Tire Fuel Efficiency Consumer Information Program (final rule)].
Other markings the sidewall must carry
Besides the size, Standard No. 139 requires each passenger and light-truck radial tire to show, among other markings:
“The tire size designation as listed in the documents and publications specified in S4.1.1 of this standard;” [49 CFR 571.139]
“The maximum permissible inflation pressure” [49 CFR 571.139]
“The maximum load rating and for LT tires, the letter designating the tire load range;” [49 CFR 571.139]
Your vehicle's original size is on the tire placard, which Standard No. 110 requires to list it [49 CFR 571.110]. The week and year a tire was made are in the date code.
Example sizes
Nominal figures for a range of passenger and light-truck sizes (examples, not a list of the most common sizes):
| Size | Sidewall | Overall diameter | Revolutions per mile |
|---|---|---|---|
| 185/65R14 | 4.73 in (120.3 mm) | 23.47 in (596 mm) | 859 |
| 195/65R15 | 4.99 in (126.8 mm) | 24.98 in (635 mm) | 807 |
| 205/55R16 | 4.44 in (112.8 mm) | 24.88 in (632 mm) | 811 |
| 205/65R16 | 5.25 in (133.3 mm) | 26.49 in (673 mm) | 761 |
| 215/55R17 | 4.66 in (118.3 mm) | 26.31 in (668 mm) | 767 |
| 215/60R16 | 5.08 in (129 mm) | 26.16 in (664 mm) | 771 |
| 225/45R17 | 3.99 in (101.3 mm) | 24.97 in (634 mm) | 808 |
| 225/60R17 | 5.31 in (135 mm) | 27.63 in (702 mm) | 730 |
| 225/65R17 | 5.76 in (146.3 mm) | 28.52 in (724 mm) | 707 |
| 235/45R18 | 4.16 in (105.8 mm) | 26.33 in (669 mm) | 766 |
| 235/65R17 | 6.01 in (152.8 mm) | 29.03 in (737 mm) | 695 |
| 245/60R18 | 5.79 in (147 mm) | 29.57 in (751 mm) | 682 |
| 255/70R18 | 7.03 in (178.5 mm) | 32.06 in (814 mm) | 629 |
| 265/70R17 | 7.3 in (185.5 mm) | 31.61 in (803 mm) | 638 |
| 275/55R20 | 5.95 in (151.3 mm) | 31.91 in (810 mm) | 632 |
| 275/60R20 | 6.5 in (165 mm) | 32.99 in (838 mm) | 611 |
Questions
What do the numbers in a tire size mean?
In a size such as 185/65R14, NHTSA explains, 185 is the nominal width in millimeters from sidewall edge to sidewall edge, 65 is the aspect ratio (the ratio of tire height to tire width), R means radial construction, and 14 is the rim diameter in inches.
How is overall tire diameter calculated?
Sidewall height is the width times the aspect ratio divided by 100 (215 × 65 / 100 = 139.75 mm, or 5.50 inches). The overall diameter is the rim diameter plus two sidewalls: 16 + 2 × 5.50 = 27.0 inches for a 215/65R16. These are nominal figures; a real tire differs by model.
How much bigger or smaller can a replacement tire be?
BFGoodrich’s guideline for a change of size is to keep the overall diameter within about 3 percent of the original; beyond that, it says, speedometer and odometer error becomes significant and ABS and stability control calibration is affected. NHTSA notes that a change in any of the three numbers is a different size specification, so use the size your vehicle maker specifies unless a tire professional approves another.
Does a bigger tire make my speedometer read slow?
Yes. A speedometer counts wheel turns, so on a tire that is 2 percent larger in diameter you travel 2 percent farther per turn: when it reads 60 mph you are doing about 61.2 mph, and the odometer under-counts by the same share.
Sources
- Tire Fuel Efficiency Consumer Information Program (final rule), 75 FR 15894. Federal Register, National Highway Traffic Safety Administration, March 30, 2010. Read October 6, 2026.
- 49 CFR 571.139, Standard No. 139: New pneumatic radial tires for light vehicles. Electronic Code of Federal Regulations (eCFR), National Highway Traffic Safety Administration rule. Read October 6, 2026.
- How to read tire size. Bridgestone Americas (tires.bridgestone.com), tire maker. Read October 7, 2026.
- Changing tire size. BFGoodrich Tires (bfgoodrichtires.com), tire maker. Read October 7, 2026.
- 49 CFR 571.110, Standard No. 110: Tire selection and rims for motor vehicles with a GVWR of 10,000 pounds or less. Electronic Code of Federal Regulations (eCFR), National Highway Traffic Safety Administration rule. Read October 7, 2026.