Enter the three numbers from your tyre sidewall — width, aspect ratio, and rim diameter — and this calculator gives you the overall diameter, sidewall height, circumference, and revs per mile. It also compares a second tyre size and tells you the speedometer impact, which is the number most people forget to check before fitting different tyres.
Tyre Size Calculator
Read the numbers from your tyre sidewall: e.g. 225/45R17 = Width 225, Aspect 45, Rim 17
How to Read a Tyre Size
Every tyre has its size printed on the sidewall in a standard format. For example: 225/45R17
- 225 — Section width in millimetres (the tyre’s widest point)
- 45 — Aspect ratio as a percentage of width (sidewall height = 225 × 0.45 = 101.25mm)
- R — Radial construction (virtually all modern tyres)
- 17 — Rim diameter in inches
The overall diameter is what matters for fitment, speedometer accuracy, and whether the tyre will clear your wheel arch and suspension components.
Common OEM Tyre Sizes and Their Dimensions
| Tyre Size | Overall Diameter | Sidewall Height | Revs/Mile |
|---|---|---|---|
| 195/65R15 | 635mm / 25.0″ | 127mm / 5.0″ | 836 |
| 205/55R16 | 632mm / 24.9″ | 113mm / 4.4″ | 840 |
| 225/45R17 | 632mm / 24.9″ | 101mm / 4.0″ | 840 |
| 235/45R18 | 657mm / 25.9″ | 106mm / 4.2″ | 808 |
| 265/70R17 | 779mm / 30.7″ | 186mm / 7.3″ | 682 |
| 275/55R20 | 813mm / 32.0″ | 151mm / 6.0″ | 654 |
| 285/75R16 | 831mm / 32.7″ | 214mm / 8.4″ | 640 |
Why Overall Diameter Matters More Than Width
Most people focus on width when changing tyre sizes, but diameter is the number that has the most consequences. Fitting a tyre with a significantly different diameter than stock affects three things:
Speedometer Accuracy
Your speedometer is calibrated to the revolutions per mile of your OEM tyre. A larger diameter tyre has fewer revs per mile — so if your speedometer says 60 mph, you might actually be doing 63 mph. The calculator’s comparison tool shows this error exactly. A difference of more than 3% is worth addressing with a speedometer recalibration.
Odometer and Fuel Economy Readings
Since both are based on wheel revolutions, a larger tyre makes your odometer read fewer miles than you actually drove. Over time this understates your service intervals. Your calculated MPG will also be artificially inflated — the car thinks it has travelled less than it actually has.
Physical Clearance
A significantly larger tyre may rub against the wheel arch, suspension components, or steering stops. Always check clearance at full lock in both directions and over full suspension compression before committing to an upsized tyre.
Plus-Sizing: Wider Rims, Lower Profile Tyres
Plus-sizing means going to a larger rim diameter while reducing the aspect ratio to keep overall diameter roughly the same. For example, 205/55R16 and 225/45R17 have nearly identical overall diameters (632mm vs 632mm), so swapping between them has almost no speedometer impact. This is the correct way to upgrade rim size without affecting driving dynamics or speedometer calibration.
Mechanic’s Tip
Keep overall diameter within 3% of stock. That is the rule I give every customer asking about upsizing. Beyond 3% you start accumulating real-world problems: speedometer error that could cost you a speeding ticket, ABS calibration issues, potential rubbing, and inaccurate service interval tracking. Use the comparison tool above — if the difference column shows more than 19mm either way on a stock 632mm tyre, reconsider the size.
Changing to a larger tyre also affects fuel economy — use the Fuel Cost Calculator to estimate the difference a heavier, wider tyre makes to your annual fuel spend.
Frequently Asked Questions
Can I put wider tyres on without changing the rim?
Up to a point. Each rim width has a range of compatible tyre widths — typically the tyre width should be 1.5-2.5x the rim width in inches. Going significantly wider than the rim supports causes the tyre to bulge outward, which affects handling and tyre wear patterns. The calculator gives you geometry but check a tyre fitment guide for your specific rim width.
Does tyre width affect fuel economy?
Yes — wider tyres have more rolling resistance and frontal area, which increases fuel consumption slightly. The effect is usually small (1-3%) for modest width increases, but it adds up over the life of the tyre.
What does the load index and speed rating mean?
After the tyre size you will see a load index and speed rating — for example 94W. The load index (94) indicates maximum load capacity per tyre. The speed rating (W) indicates maximum sustained speed. Never fit a tyre with a lower load index or speed rating than OEM spec — it is a safety issue, not just a recommendation.
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