Tire Size Calculator
Enter a P-metric tire size such as 225/45R17 to get overall diameter, sidewall height, circumference and revolutions per mile. Switch on comparison mode to put a second size beside the first and see the speedometer correction at every highway speed, plus a diameter-difference warning.
Formula
Worked example
For 225/45R17: sidewall = 225 x 0.45 = 101.25 mm. Diameter = 17 x 25.4 + 2 x 101.25 = 431.8 + 202.5 = 634.3 mm (24.97 in). Circumference = pi x 634.3 = 1992.6 mm (78.45 in). Revolutions per mile = 1,609,344 / 1992.6 = 808. Comparing to 235/40R18: diameter = 18 x 25.4 + 2 x (235 x 0.40) = 457.2 + 188 = 645.2 mm (25.40 in). Difference = (645.2 - 634.3) / 634.3 x 100 = +1.72%, within the 3% safe zone.
How to read a P-metric tire size
A size like 225/45R17 packs three measurements into one code. The first number, 225, is the section width in millimetres, the distance across the tread. The second number, 45, is the aspect ratio: the sidewall height expressed as a percentage of that width, so the sidewall here is 45% of 225 mm = 101.25 mm. The R means radial construction, and 17 is the wheel rim diameter in inches. Because the size mixes millimetres and inches, the calculator converts everything to millimetres first, then offers results in both systems.
Why overall diameter matters when changing sizes
The overall diameter sets how far your car travels per wheel revolution, which is exactly what the speedometer and odometer assume. Fit a taller tire and the car will actually be going faster than the dash reads; fit a shorter one and the dash reads high. The rule of thumb shared by tire manufacturers and OEM engineers is to keep a replacement size within about 3% of the original diameter: within that window the speedometer error is typically under 2 mph at highway speeds, gearing feels right, and clearance issues are rare. The comparison mode here shows the diameter percentage difference and the exact speedometer correction at any reference speed, so you can make the call with real numbers rather than guessing.
Sidewall height, ride feel, and plus-sizing
The aspect ratio is the single biggest driver of how a tire feels on the road. A low aspect ratio, 40 or below, gives a short, stiff sidewall that sharpens steering response and looks sporty, but transmits more bumps and is more vulnerable to pothole damage. A taller sidewall, 60 and up, cushions the ride and protects the wheel rim, at the cost of a little steering precision. "Plus-sizing" means stepping up to a bigger wheel diameter while choosing a lower aspect ratio to keep the overall diameter close to stock: for example, moving from 205/55R16 to 225/45R17. Both tires are about 24.9 inches tall, so the speedometer stays honest while the wider stance and larger wheel give a sportier look.
Speedometer error and the correction table
Your speedometer reads revolutions from the ABS wheel-speed sensors or a traditional transmission speed sensor, then converts that to miles per hour using the circumference programmed at the factory. Swap to a tire with a different circumference and every speed reading shifts by the same ratio: actual speed = indicated speed times (new circumference / original circumference). The schedule table in comparison mode runs this correction across the most common highway speeds, from 20 mph up to 90 mph, so you can see whether the error is trivial (under 1 mph) or significant (2 mph or more at cruise speeds, which can matter for speed limits and toll road cameras).
Common tire sizes and their key dimensions
| Tire size | Sidewall (in) | Overall diameter (in) | Circumference (in) | Revs per mile |
|---|---|---|---|---|
| 195/65R15 | 4.99 | 24.98 | 78.45 | 807 |
| 205/55R16 | 4.44 | 24.88 | 78.13 | 811 |
| 225/45R17 | 3.99 | 24.97 | 78.43 | 808 |
| 235/40R18 | 3.70 | 25.40 | 79.79 | 794 |
| 245/35R19 | 3.38 | 25.75 | 80.88 | 783 |
| 255/30R20 | 3.01 | 26.01 | 81.72 | 775 |
| 265/70R17 | 7.30 | 31.61 | 99.29 | 638 |
| 275/65R18 | 7.04 | 32.07 | 100.74 | 629 |
Computed from section width, aspect ratio, and rim diameter.
Frequently asked questions
What does 225/45R17 mean?
225 is the tread (section) width in millimetres, 45 is the aspect ratio (the sidewall is 45% of the width, so 101.25 mm tall), R means radial construction, and 17 is the wheel rim diameter in inches. Multiplying it out: diameter = 17 x 25.4 + 2 x 101.25 = 634.3 mm, about 24.97 inches.
How much can I change tire size without affecting my speedometer?
Keep the new tire's overall diameter within about 3% of the original. Beyond that, the speedometer and odometer readings drift noticeably. For example, a 3% taller tire makes the speedometer read about 3% low: if the dash says 60 mph you are actually doing roughly 61.8 mph. Use comparison mode and the speedometer correction table to see the exact error for your specific sizes.
Does a higher aspect ratio mean a bigger tire?
Not necessarily. A higher aspect ratio means a taller sidewall relative to the width, but overall diameter also depends on the section width and rim diameter. A 235/35R19 and a 205/55R16 can both measure close to 25 inches tall despite very different aspect ratios.
What is plus-sizing in tires?
Plus-sizing means fitting a larger-diameter wheel while choosing a lower-profile (lower aspect ratio) tire to keep the overall tire diameter close to stock. For example, going from 205/55R16 (24.88 in) to 225/45R17 (24.97 in) is a "+1" fitment: the wheel grew one inch, the aspect ratio dropped from 55 to 45, and the overall diameter barely changed. The benefit is a stiffer, more responsive sidewall and room for larger brakes.
How do I calculate speedometer error after changing tire size?
Actual speed = indicated speed x (new tire circumference / original tire circumference). If you move from a 225/45R17 (circumference 1992.6 mm) to a 235/40R18 (circumference 2026.8 mm), the ratio is 1.0172. At an indicated 60 mph you are actually travelling 60 x 1.0172 = 61.0 mph. The comparison mode on this calculator builds a full correction table across highway speeds so you do not have to do this manually.
What is the difference between tire diameter and rim diameter?
Rim diameter is the steel or alloy wheel, measured across the bead seat where the tire mounts. Tire overall diameter is the rim plus two sidewalls: one on each side. A 225/45R17 tire has a 17-inch rim but a 24.97-inch overall diameter because the two 4-inch sidewalls add 8 inches to the wheel.