Tire Size Calculator
Compare your current and new tire sizes to see the diameter difference, true speed behind each speedometer reading, sidewall height and revolutions per mile.
Flotation sizes (optional)
Inch sizes such as 33x12.50R15: overall diameter × width, then rim. A value here replaces the metric fields for that tire.
At an indicated 60 mph you would travel 61.0 mph.
Diameter change in percent, from −5 to +5. Sumitomo's plus sizing guideline is within 2%, with 1% as the optimum.
Speedometer readings
For every 100 miles on the odometer you would cover 101.7 miles.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 mph |
| 45 mph | 45.8 mph | +0.8 mph |
| 55 mph | 55.9 mph | +0.9 mph |
| 65 mph | 66.1 mph | +1.1 mph |
| 75 mph | 76.3 mph | +1.3 mph |
| 80 mph | 81.4 mph | +1.4 mph |
Your calculation
The same steps the calculator runs, with your sizes.
- Current sidewall: 225 × 45 ÷ 100 = 101.25 mm.
- Current diameter: 17 × 25.4 + 2 × 101.25 = 634.3 mm = 24.97 in.
- Current revolutions per mile: 63,360 ÷ (π × 24.97) = 63,360 ÷ 78.45 = 807.6.
- New sidewall: 235 × 40 ÷ 100 = 94 mm.
- New diameter: 18 × 25.4 + 2 × 94 = 645.2 mm = 25.40 in.
- New revolutions per mile: 63,360 ÷ (π × 25.40) = 63,360 ÷ 79.80 = 794.0.
- Diameter change: (645.2 ÷ 634.3 − 1) × 100 = +1.72%.
- Actual speed: 60 × 645.2 ÷ 634.3 = 61.03 mph.
- Gearing factor: 634.3 ÷ 645.2 = 0.9831.
Side by side comparison
| Measure | Current | New |
|---|---|---|
| Size | 225/45R17 | 235/40R18 |
| Type | Euro metric passenger | Euro metric passenger |
| Section width | 225 mm (8.86 in) | 235 mm (9.25 in) |
| Aspect ratio | 45% | 40% |
| Sidewall height | 101.3 mm (3.99 in) | 94.0 mm (3.70 in) |
| Rim diameter | 17 in (431.8 mm) | 18 in (457.2 mm) |
| Overall diameter | 24.97 in (634.3 mm) | 25.40 in (645.2 mm) |
| Circumference | 78.45 in (1,992.7 mm) | 79.80 in (2,027.0 mm) |
| Revs per mile | 807.6 | 794.0 |
| Revs per km | 501.8 | 493.4 |
Drawn to scale. The dashed line marks the top of the taller tire; the light circles are the rims.
Similar diameter on other rims
Sizes on other rims within 3% of your current 225/45R17. Confirm the load index, rim width and clearance before buying.
| Size | Diameter | vs current |
|---|---|---|
| 205/55R16 | 24.88 in | −0.38% |
| 225/40R18 | 25.09 in | +0.46% |
| 245/30R19 | 24.79 in | −0.74% |
Calculated from nominal sizes. Measured dimensions vary by brand, tread depth and load, so follow the vehicle placard and a qualified tire professional.
How the Tire Size Calculator Works
This tire size calculator compares the tire on your vehicle now with the size you are thinking of buying. Enter each size the way it is molded on the sidewall, such as 225/45R17, P225/45ZR17 or LT265/75R16, and the calculator works out the sidewall height, overall diameter, circumference and revolutions per mile of both tires. It then reports the percentage difference in diameter, the true speed behind each speedometer reading, and how ground clearance and gearing change.
Every result comes from the nominal dimensions in the size code. The formulas below are the ones the calculator runs, shown with the default comparison of 225/45R17 against 235/40R18.
The 63,360 in the revolutions formula is the number of inches in one mile (5,280 feet × 12). Values are rounded only for display, so percentages and speeds use the full precision of each diameter.
How to Read a Tire Size
A metric tire size has six parts. Take P225/45R17 91W as an example:
- P is the tire type. P marks a P metric passenger tire, a designation standardized by the Tire and Rim Association. No letter means a Euro metric size, standardized by the European Tyre and Rim Technical Organisation (ETRTO). LT means light truck, ST special trailer and T temporary spare.
- 225 is the nominal section width in millimeters, measured from sidewall to sidewall.
- 45 is the aspect ratio: the sidewall height as a percentage of the width. 45 percent of 225 mm is 101.25 mm.
- R means radial construction. D or B marks bias ply, RF a run flat tire, and ZR a tire rated above 149 mph.
- 17 is the rim diameter in inches. A different wheel diameter always needs a tire made for it.
- 91W is the service description: load index 91 and speed rating W.
The size your vehicle needs is printed on the Tire and Loading Information placard on the driver's door edge or post, and in the owner's manual. NHTSA advises buying tires that are the same size as the original tires or another size recommended by the vehicle manufacturer.
Load index and speed rating
The load index is a two or three digit code for the weight each tire can support, and the speed rating is a letter for the speed the tire is designed to sustain. Bridgestone notes that load index numbers from 1 to 150 cover 99 to 7,385 lb, and that P metric and Euro metric tires with the same index can carry different loads, so check the actual capacity. Bridgestone also states that replacement tires must have the same or higher speed rating as the original equipment.
| Speed rating | Q | R | S | T | U | H | V | W | Y |
|---|---|---|---|---|---|---|---|---|---|
| Maximum mph | 99 | 106 | 112 | 118 | 124 | 130 | 149 | 168 | 186 |
The ratings come from NHTSA's tire safety booklet. Tires able to exceed 149 mph sometimes carry ZR in the size, and tires able to exceed 186 mph always do.
Other sidewall markings
The DOT Tire Identification Number begins with DOT and a plant code, and its last four digits give the week and year of manufacture, so 3197 means the 31st week of 1997. UTQG grades rate treadwear (a 400 tire should wear twice as long as a 200 tire on the government test course), wet traction (AA, A, B or C) and heat resistance (A, B or C). The maximum permissible inflation pressure on the sidewall is a limit, not the pressure to run: the vehicle placard gives the correct cold pressure.
Tire Size Chart: Diameter of Common Sizes
This tire size chart lists the calculated sidewall height, overall diameter and revolutions per mile of common car, SUV and truck sizes. Sizes within a few tenths of an inch of each other are the usual candidates to swap, but load index and clearance still decide the fit.
| Size | Sidewall | Diameter | Revs per mile |
|---|---|---|---|
| 195/65R15 | 126.8 mm (4.99 in) | 24.98 in (635 mm) | 807 |
| 205/55R16 | 112.8 mm (4.44 in) | 24.88 in (632 mm) | 811 |
| 215/55R17 | 118.3 mm (4.66 in) | 26.31 in (668 mm) | 767 |
| 225/45R17 | 101.3 mm (3.99 in) | 24.97 in (634 mm) | 808 |
| 225/45R18 | 101.3 mm (3.99 in) | 25.97 in (660 mm) | 777 |
| 225/65R17 | 146.3 mm (5.76 in) | 28.52 in (724 mm) | 707 |
| 235/40R18 | 94.0 mm (3.70 in) | 25.40 in (645 mm) | 794 |
| 235/65R17 | 152.8 mm (6.01 in) | 29.03 in (737 mm) | 695 |
| 245/45R18 | 110.3 mm (4.34 in) | 26.68 in (678 mm) | 756 |
| 255/55R18 | 140.3 mm (5.52 in) | 29.04 in (738 mm) | 694 |
| 265/70R17 | 185.5 mm (7.30 in) | 31.61 in (803 mm) | 638 |
| 265/75R16 | 198.8 mm (7.82 in) | 31.65 in (804 mm) | 637 |
| 285/70R17 | 199.5 mm (7.85 in) | 32.71 in (831 mm) | 617 |
| 285/75R16 | 213.8 mm (8.42 in) | 32.83 in (834 mm) | 614 |
| 315/70R17 | 220.5 mm (8.68 in) | 34.36 in (873 mm) | 587 |
| 31x10.50R15 | 203.2 mm (8.00 in) | 31.00 in (787 mm) | 651 |
| 33x12.50R15 | 228.6 mm (9.00 in) | 33.00 in (838 mm) | 611 |
| 35x12.50R17 | 228.6 mm (9.00 in) | 35.00 in (889 mm) | 576 |
Flotation sizes state the diameter directly, so 33x12.50R15 and 33x12.50R17 share the same 33 inch diameter. The 17 inch version simply has a shorter 8 inch sidewall.
Speedometer Error from Bigger or Smaller Tires
A speedometer converts wheel or driveline rotation into speed using a fixed revolutions per mile figure for the original tire. A taller tire covers more ground per turn, so the vehicle travels faster than the speedometer shows, and a shorter tire does the opposite. The error in percent equals the diameter change in percent, and the odometer is off by the same amount.
| Size change | Diameter change | At 60 mph shown | At 100 km/h shown |
|---|---|---|---|
| 195/65R15 to 205/55R16 | −0.41% | 59.75 mph | 99.59 km/h |
| 225/45R18 to 215/55R17 | +1.30% | 60.78 mph | 101.30 km/h |
| 225/45R17 to 235/40R18 | +1.72% | 61.03 mph | 101.72 km/h |
| 225/45R17 to 245/45R17 | +2.84% | 61.70 mph | 102.84 km/h |
| 265/70R17 to 285/70R17 | +3.49% | 62.09 mph | 103.49 km/h |
| 225/70R15 to 235/75R15 | +5.39% | 63.23 mph | 105.39 km/h |
Some vehicles let a dealer reprogram the tire size so the speedometer reads correctly again. Sumitomo's plus sizing guidelines advise contacting the vehicle dealer to reset the electronic codes for affected systems, because ABS, engine management, transmission shift points and cruise control all use tire revolutions per mile.
How a bigger tire changes gearing
A taller tire acts like a numerically lower axle ratio. The effective ratio is the axle ratio × old diameter ÷ new diameter. With a 3.73 axle, moving from 265/70R17 to 285/70R17 gives 3.73 × 802.8 ÷ 830.8 = 3.604, so the engine turns fewer rpm at a given road speed and acceleration feels a little softer. Matching the original feel would take an axle near 3.73 × 830.8 ÷ 802.8 = 3.860.
Plus Sizing: How Much Diameter Change Is Acceptable?
Plus sizing means fitting a wheel one or more inches larger with a lower profile tire, so the overall diameter stays close to the original. Going from 225/45R17 to 235/40R18 is plus one: the rim grows by 25.4 mm, the sidewall shrinks from 101.25 to 94 mm, and the overall diameter rises by 10.9 mm. Fitting a smaller wheel with a taller sidewall, common for winter tires, is called minus sizing.
No single legal limit applies. Sumitomo's plus sizing guidelines recommend an overall diameter within 2 percent of the original equipment fitment, call 1 percent optimal, and say the shorter option is usually better for stability, turning, acceleration and braking. In Modern Tire Dealer, plus sizing consultant Joe Findeis says it is widely accepted that a variance of 2 to 3 percent or less should pose no adverse effects on vehicle systems. The badge in the results uses these bands.
| Diameter change | Badge | Error at 60 mph | Guidance |
|---|---|---|---|
| Up to 1% | Within 1% | 0.6 mph or less | Sumitomo's optimum |
| 1 to 2% | Within 2% | 0.6 to 1.2 mph | Inside Sumitomo's 2 percent guideline |
| 2 to 3% | Within 3% | 1.2 to 1.8 mph | The 2 to 3 percent range cited in Modern Tire Dealer |
| Over 3% | Over 3% | More than 1.8 mph | Outside the cited guidance; check the owner's manual |
Diameter is only one check. Sumitomo also lists load capacity, inflation pressure, an equal or higher speed rating, rim width and offset, wheel weight, brake capacity and clearance in the wheel well through steering lock, bumps and dips. Its guidance says not to fit a plus size with a lower load index than the original equipment tire.
LT, P Metric and Flotation Tire Sizes
P metric and Euro metric tires are passenger tires. LT metric tires, such as LT265/75R16, are built for heavier light trucks and show two load indexes, one for single and one for dual rear wheels, plus a load range letter. The diameter formula is the same for every metric type, so 265/75R16 and LT265/75R16 both calculate to 31.65 inches.
Flotation sizes are written in inches. In 33x12.50R15 the 33 is the overall diameter, 12.50 the section width and 15 the rim diameter. The calculator converts them: the sidewall is (33 − 15) ÷ 2 = 9 inches, the width is 12.5 × 25.4 = 317.5 mm, and the equivalent aspect ratio is 9 ÷ 12.5 = 72 percent. A 285/75R16 calculates to 32.83 inches, 0.51 percent smaller than the 33 inch tire.
Bridgestone notes that a flotation tire may have a different load capacity than its LT metric counterpart and cannot be used in dual applications. The calculator shows a reminder when you compare a passenger tire with a light truck tire.
Revolutions per Mile: Calculated vs Measured
Calculated revolutions per mile assume the tire rolls on its full unloaded radius. On a vehicle the tread flattens where it meets the road, so the effective rolling radius is shorter than half the overall diameter and the tire turns slightly more often than the formula predicts. Tread depth, inflation pressure, load and brand also move the number.
That is why tire makers measure rolling circumference instead of calculating it. ISO 17269 specifies a drum test and a road test for measuring the rolling circumference and revolutions per kilometer of loaded new passenger tires. Use the calculator to compare sizes, and the tire maker's published revolutions per mile to recalibrate a speedometer.
How to Use the Calculator
- Current tire. Type the size from the sidewall or door placard, or fill in width, aspect ratio and rim.
- New tire. The size you plan to buy. Each size field accepts P, LT and Euro metric sizes.
- Flotation sizes. Type 33x12.50R15 in a size field or in the optional section.
- Speed units. Choose mph or km/h for the speedometer table.
- Swap tires. Reverses the comparison. Example buttons fill both sizes.
Common Mistakes
- Reading the aspect ratio as millimeters. 45 means 45 percent of the width, not 45 mm.
- Entering the tires backward. Every sign flips; use Swap tires to correct it.
- Comparing with a worn tire. New tread is deeper, so a measured old tire is shorter than its size suggests.
- Ignoring the load index. A matching diameter says nothing about load capacity.
- Using the sidewall pressure. It is a maximum. The placard gives the correct pressure.
Frequently Asked Questions
What does 225/45R17 mean on a tire?
225/45R17 describes a tire with a nominal section width of 225 millimeters, measured from sidewall to sidewall. The 45 is the aspect ratio: the sidewall height is 45 percent of the width, or 101.25 mm. R means radial construction, and 17 is the rim diameter in inches. Adding the rim (17 × 25.4 = 431.8 mm) to two sidewalls gives an overall diameter of 634.3 mm, or 24.97 inches. A P before the width marks a P metric passenger tire, LT marks a light truck tire, and no letter means a Euro metric size. Characters after the size, such as 91W, are the load index and speed rating.
How do you calculate tire diameter from the tire size?
Multiply the section width by the aspect ratio and divide by 100 to get the sidewall height in millimeters. Double it, because the tire has a sidewall above and below the rim, and add the rim diameter converted to millimeters (inches × 25.4). For 235/40R18: 235 × 40 ÷ 100 = 94 mm, then 18 × 25.4 + 2 × 94 = 645.2 mm. Divide by 25.4 to get 25.40 inches. For a flotation size such as 33x12.50R15, the first number already is the overall diameter, 33 inches.
How much do bigger tires affect the speedometer?
The speedometer counts wheel revolutions calibrated for the original tire, so the error equals the percentage change in diameter. Actual speed = indicated speed × new diameter ÷ old diameter. Going from 265/70R17 (31.61 in) to 285/70R17 (32.71 in) is 3.49 percent taller, so an indicated 60 mph is really 62.1 mph and 100 km/h is 103.5 km/h. A taller tire makes the speedometer and odometer read low, and a shorter tire makes them read high. A plus one change from 225/45R17 to 235/40R18 is 1.72 percent taller, which turns 60 mph into 61.0 mph.
Can I put bigger tires on my car?
Often, but check three things first. NHTSA advises buying tires that match the original size or another size the vehicle manufacturer recommends, listed on the door placard or in the owner's manual. Sumitomo's plus sizing guidelines keep the overall diameter within 2 percent of the original fitment, with 1 percent as the optimum, because ABS, stability control, transmission shift points and the speedometer use tire revolutions per mile. The replacement must also carry at least the original load index and speed rating, and it must clear the body, suspension and brakes through turns and bumps. A tire professional can confirm the fit.
What does 33x12.50R15 mean?
33x12.50R15 is a flotation size, used mostly on off road trucks and SUVs. The first number is the overall diameter in inches (33), the second is the section width in inches (12.50), R means radial, and 15 is the rim diameter in inches. The sidewall is (33 − 15) ÷ 2 = 9 inches, or 228.6 mm, and the width is 317.5 mm. A common metric alternative is 285/75R16, which calculates to 32.83 inches, 0.51 percent smaller. Flotation tires may carry a different load capacity than LT metric tires of similar size, so compare the load index too.
How do you calculate tire revolutions per mile?
Divide the 63,360 inches in a mile by the tire's circumference in inches, where circumference = π × overall diameter. A 225/45R17 tire is 24.97 inches tall, so its circumference is 78.45 inches and it turns 807.6 times per mile. In metric, divide 1,000,000 mm by the circumference in millimeters: 1,000,000 ÷ 1,992.7 = 501.8 revolutions per kilometer. Figures published by tire makers can differ slightly, because a loaded tire deflects where it meets the road and rolls on a shorter radius. ISO 17269 sets out how rolling circumference is measured on a loaded tire.
Sources & References
- NHTSA. Tires (TireWise): tire size, UTQG grades, DOT date code. nhtsa.gov
- NHTSA. Tire Safety: Everything Rides On It, DOT HS 809 361: sidewall markings, load index, speed ratings. nhtsa.gov
- Bridgestone Americas. How to Read & Determine Tire Size for Your Vehicle. bridgestoneamericas.com
- Sumitomo Tire. Plus Sizing Guidelines, Technical Data, 2018 consumer catalog. sumitomotires.com
- Manges, M. The ABCs of Plus Sizing. Modern Tire Dealer, July 6, 2022. moderntiredealer.com
- ISO. ISO 17269:2000 Passenger car tyres: Methods for measuring rolling circumference, Loaded new tyres, confirmed 2022. iso.org
- The Tire and Rim Association (P metric and LT metric standards) and the European Tyre and Rim Technical Organisation (Euro metric standards).
Results are estimates calculated from nominal tire sizes. Follow the vehicle placard and owner's manual, and have a qualified tire professional confirm fitment. Every number on this page was recomputed with the formulas the calculator uses. Methodology reviewed: 2026.