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Tire Size Calculator
Enter a tire size to get its overall diameter, sidewall height, section width, circumference, and revolutions per mile — in inches and millimetres.
Short answer
A tire size like 275/40R20 means a 275 mm section width, a sidewall that's 40% of that width, on a 20-inch wheel. Overall diameter is the wheel diameter plus twice the sidewall height — here about 28.7 inches. Enter any size below for its full dimensions, or compare two sizes to see the speedometer error.
275/40R20
- Overall diameter
- 28.66" · 728 mm
- Sidewall height
- 4.33" · 110 mm
- Section width
- 10.83" · 275 mm
- Circumference
- 90.04"
- Revolutions / mile
- 704
Tire tools
Tire size chart — common sizes
| Size | Diameter | Sidewall | Section width | Revs/mile |
|---|---|---|---|---|
| 185/65R15 | 24.47" | 4.73" | 7.28" | 824 |
| 195/65R15 | 24.98" | 4.99" | 7.68" | 807 |
| 205/55R16 | 24.88" | 4.44" | 8.07" | 811 |
| 215/60R16 | 26.16" | 5.08" | 8.46" | 771 |
| 225/45R17 | 24.97" | 3.99" | 8.86" | 808 |
| 235/45R18 | 26.33" | 4.16" | 9.25" | 766 |
| 245/40R18 | 25.72" | 3.86" | 9.65" | 784 |
| 255/35R19 | 26.03" | 3.51" | 10.04" | 775 |
| 265/70R17 | 31.61" | 7.30" | 10.43" | 638 |
| 275/40R20 | 28.66" | 4.33" | 10.83" | 704 |
| 285/45R22 | 32.10" | 5.05" | 11.22" | 628 |
| 305/35R20 | 28.41" | 4.20" | 12.01" | 710 |
Overall diameter = wheel diameter + 2 × sidewall height, where sidewall = section width × aspect ratio. Computed from this page's tire engine; actual mounted dimensions vary slightly by brand and inflation.
How tire sizes work
The three parts of a size such as 275/40R20 are the section width in millimetres (275), the aspect ratio — sidewall height as a percentage of width (40) — and the wheel diameter in inches (20). The overall diameter that matters for speedometer accuracy and fitment is the wheel diameter plus twice the sidewall height. Change a size and keep the diameter within about 3% to avoid speedometer error and clearance issues.
How we calculate this
Every dimension comes from the same geometry the calculator uses — no lookup tables, just the tire size definition:
- Sidewall height. section width (mm) × aspect ratio ÷ 100 gives the sidewall in millimetres; divide by 25.4 for inches. Example: 245/45R18 → 245 × 0.45 = 110.25 mm ≈ 4.34 in.
- Overall diameter. wheel diameter (inches) + 2 × sidewall height (inches). This is the number that drives speedometer accuracy and fitment.
- Revolutions per mile. 63,360 inches per mile ÷ (π × overall diameter). A larger diameter means fewer revolutions per mile — and a slower-reading speedometer.
- Speedometer error. a speedometer calibrated for the old tire reads indicated speed × (new diameter ÷ old diameter). A taller tire means you're going faster than it shows.
Assumptions
- Nominal dimensions from the tire size code — real mounted diameter varies slightly with brand, load, and inflation pressure.
- Metric (P-metric) sizing; flotation sizes are converted via overall diameter.
- Educational estimates for fitment planning, not a substitute for a fitment guarantee from a tire retailer.
Last reviewed: July 17, 2026
Frequently asked questions
Related tools
What do the numbers in a tire size mean?+
In a size like 245/45R18, the first number (245) is the section width in millimetres. The second (45) is the aspect ratio — the sidewall height as a percentage of the width, so 45% of 245 mm here. The R means radial construction, and the last number (18) is the wheel diameter in inches. Overall tire diameter is the wheel diameter plus twice the sidewall height.
How do I calculate a tire's overall diameter?+
Multiply the width by the aspect ratio to get the sidewall height in millimetres (e.g. 245 × 0.45 = 110.25 mm), divide by 25.4 to convert to inches (about 4.34 in), then add twice the sidewall to the wheel diameter: 18 + 2 × 4.34 ≈ 26.68 inches. This calculator does it for you and also gives circumference and revolutions per mile.
How much bigger tire can I fit without affecting my speedometer?+
As a rule of thumb, keep the new tire's overall diameter within about 3% of the original. Beyond that, your speedometer and odometer error grows, and you may run into clearance or gearing problems. Use the comparison tool to check the diameter difference and exact speedometer error before changing sizes.
How does a different tire size affect my speedometer?+
Your speedometer counts wheel revolutions and assumes your original tire diameter. A larger-diameter tire covers more distance per revolution, so you're actually going faster than the speedometer shows and your odometer under-counts miles. A smaller tire does the opposite. The comparison tool shows your exact speed at an indicated 60 mph.
What does 'plus sizing' a tire mean?+
Plus sizing means fitting a larger-diameter wheel with a lower-profile tire so the overall diameter stays roughly the same. For each inch of wheel diameter you add, you drop the aspect ratio to keep the tire's outside diameter constant — preserving speedometer accuracy while improving handling and looks. Compare your current and target sizes above to keep the diameter within a few percent.
Are metric and inch (flotation) tire sizes interchangeable?+
They describe the same tire two ways. Metric sizes (275/40R20) use width in millimetres and an aspect ratio; flotation sizes (33x12.50R20) state the overall diameter and width in inches directly. Converting the metric size to inches lets you compare them — this tool shows overall diameter and width in both units.
Dimensions are calculated from the nominal tire size; actual mounted size varies slightly by brand, load, and inflation pressure. For fitment and load/speed ratings, confirm with a tire professional before purchasing.