Tire Size Calculator
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235/40R18 is +1.72% in diameter compared with 225/45R17; at 60 mph indicated you would be doing 61.0 mph.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 25 mph | 25.4 mph | +0.4 mph |
| 35 mph | 35.6 mph | +0.6 mph |
| 45 mph | 45.8 mph | +0.8 mph |
| 55 mph | 55.9 mph | +0.9 mph |
| 60 mph | 61.0 mph | +1.0 mph |
| 65 mph | 66.1 mph | +1.1 mph |
| 70 mph | 71.2 mph | +1.2 mph |
| 75 mph | 76.3 mph | +1.3 mph |
| 80 mph | 81.4 mph | +1.4 mph |
Within the ±3% diameter rule of thumb. That is a common guide, not an approval: choose sizes listed by your vehicle manufacturer, with a load index and speed rating at least as high as the original.
How to use the tire size calculator
- Type the size on your current tires, for example 225/45R17. A P or LT prefix and the load and speed code (94W) can be included or left off.
- Type the size you are considering in the second box, or leave it blank to see the dimensions of one tire.
- Switch between US / Imperial (inches, mph, revolutions per mile) and Metric (mm, km/h, revolutions per km).
- Check the diameter difference and the speedometer table before you buy.
Tire size formula
Sidewall = W × A ÷ 100 · Diameter = 2 × sidewall + R × 25.4 mm
Circumference = π × diameter · Revolutions per mile = 1,609,344 mm ÷ circumference
Actual speed = indicated speed × new diameter ÷ original diameter
The first number is the section width in millimeters, the second is the aspect ratio (the sidewall height as a percentage of the width) and the number after R (radial) is the rim diameter in inches. The overall diameter is two sidewalls plus the rim. These are nominal figures from the size marking: real tires differ a little by brand and model and flatten slightly under load, so measured revolutions per mile are usually somewhat higher.
Worked example
A 225/45R17 tire has a sidewall of 225 × 0.45 = 101.25 mm and an overall diameter of 2 × 101.25 + 17 × 25.4 = 634.3 mm (24.97 in), turning 808 times per mile. A 235/40R18 has a 94 mm sidewall and a 645.2 mm (25.4 in) diameter, +1.72% larger. With the speedometer still set for the original size, an indicated 60 mph is really 61.0 mph, and the odometer under-reads by the same proportion.
Dimensions of common tire sizes
| Size | Sidewall | Diameter | Diameter (mm) | Revs per mile |
|---|---|---|---|---|
| 195/65R15 | 4.99 in | 24.98 in | 634.5 | 807 |
| 205/55R16 | 4.44 in | 24.88 in | 631.9 | 811 |
| 215/60R16 | 5.08 in | 26.16 in | 664.4 | 771 |
| 225/45R17 | 3.99 in | 24.97 in | 634.3 | 808 |
| 225/65R17 | 5.76 in | 28.52 in | 724.3 | 707 |
| 235/65R17 | 6.01 in | 29.03 in | 737.3 | 695 |
| 245/45R18 | 4.34 in | 26.68 in | 677.7 | 756 |
| 255/35R19 | 3.51 in | 26.03 in | 661.1 | 775 |
| 265/70R17 | 7.3 in | 31.61 in | 802.8 | 638 |
| LT265/75R16 | 7.82 in | 31.65 in | 803.9 | 637 |
| 275/55R20 | 5.95 in | 31.91 in | 810.5 | 632 |
| 275/60R20 | 6.5 in | 32.99 in | 838 | 611 |
Plus sizing: bigger wheels, same diameter
Fitting larger wheels with lower-profile tires keeps the overall diameter close to the original. Starting from 205/55R16, the engine gives these differences. Check every size against your vehicle's approved list: width, offset and load rating matter as much as diameter.
| Size | Diameter | Difference | Actual speed at 60 mph shown |
|---|---|---|---|
| 205/55R16 | 24.88 in | Original | 60.0 mph |
| 215/50R17 | 25.46 in | +2.36% | 61.4 mph |
| 225/45R17 | 24.97 in | +0.38% | 60.2 mph |
| 225/40R18 | 25.09 in | +0.84% | 60.5 mph |
| 235/40R18 | 25.4 in | +2.10% | 61.3 mph |
| 225/35R19 | 25.2 in | +1.30% | 60.8 mph |
| 235/35R19 | 25.48 in | +2.41% | 61.4 mph |
Before you change tire size
- Start from the placard. The approved sizes, load index and pressures are on the label in the driver's door jamb and in the owner's manual. Sizes outside that list may affect handling, braking and warranty or insurance.
- The ±3% figure is a rule of thumb. Many fitters use it as a guide for diameter, but it is not a legal or manufacturer limit. A smaller difference is better.
- Load and speed ratings. The new tire's load index and speed rating should be at least those of the original. LT tires use different load rules from P-metric ones.
- Speedometer rules. Vehicle rules used in Europe and many other countries (UN Regulation No. 39) require a speedometer never to show less than the true speed. A larger tire makes yours read lower than your true speed, so drive by the corrected figure.
Changing tires can change fuel economy: measure it with the gas mileage calculator and plan trips with the fuel cost calculator. For the circle math behind circumference, see the circle calculator.
Frequently asked questions
What do the numbers on a tire mean?
In 225/45R17, 225 is the section width in millimeters, 45 is the aspect ratio (the sidewall is 45% of the width, 101.25 mm), R means radial construction and 17 is the rim diameter in inches. A P prefix marks a P-metric passenger tire and LT a light-truck tire.
How do I calculate tire diameter?
Multiply the width by the aspect ratio to get the sidewall, double it and add the rim diameter converted to millimeters. For 225/45R17: 2 × 101.25 + 17 × 25.4 = 634.3 mm, or 24.97 inches.
How much does a bigger tire change my speedometer?
By the same percentage as the diameter. Going from 225/45R17 (634.3 mm) to 235/40R18 (645.2 mm) is 1.72% larger, so when the speedometer shows 60 mph you are really doing about 61.0 mph, and the odometer under-reads by the same proportion.
How much bigger can I go on tire size?
Use the sizes your vehicle manufacturer lists on the door-jamb placard or in the manual. A common rule of thumb keeps the overall diameter within about 3% of the original, with a load index and speed rating at least as high, but it is only a guide, not an approval.
How many revolutions per mile does a tire make?
Divide a mile (1,609,344 mm) by the circumference. A 225/45R17 has a nominal circumference of about 1,993 mm, so it turns about 808 times per mile. Published figures from tire makers are usually a little higher because a loaded tire is slightly flattened.