Free wheel and tyre tools
Tyre size calculator
Compare two tyre sizes for diameter, width, circumference, revolutions and speedometer changes, with a drawing at the same scale.
How it works
A tyre marked 225/45 R17 is nominally 225 mm wide, with a sidewall that is 45% of that width, on a 17 inch rim. Diameter is rim size × 25.4 + two sidewalls. Circumference is diameter × π. These are unloaded estimates; tread, pressure, load and construction affect real rolling distance.
A worked example
Changing from 225/45 R17 to 235/40 R18 takes nominal diameter from 634.3 mm to 645.2 mm, an increase of 10.9 mm or 1.72%. At an indicated 60 mph, calculated road speed becomes 61.03 mph if the original reading was accurate.
Quick reference
Common plus-one sizes and their effect
| From | To | Diameter change | Change | At an indicated 60 mph |
|---|---|---|---|---|
| 195/65 R15 | 205/55 R16 | -2.6 mm | -0.41% | 59.75 mph |
| 205/55 R16 | 225/45 R17 | +2.4 mm | +0.38% | 60.23 mph |
| 205/55 R16 | 205/50 R17 | +4.9 mm | +0.78% | 60.47 mph |
| 225/45 R17 | 225/40 R18 | +2.9 mm | +0.46% | 60.27 mph |
| 225/45 R17 | 235/40 R18 | +10.9 mm | +1.72% | 61.03 mph |
| 225/40 R18 | 235/35 R19 | +9.9 mm | +1.55% | 60.93 mph |
| 255/35 R19 | 255/30 R20 | -0.1 mm | -0.02% | 59.99 mph |
Nominal size-code calculations, assuming the speedometer was accurate on the original tyre.
Explained in our guides
- Plus-sizing wheels: tyre diameter and speedometer changesWork through a 17 to 18 inch wheel change, compare tyre diameters and speedometer effects, and understand why a 3% rule cannot approve fitment.
- How to read a tyre size, load index and sidewall markingsDecode tyre width, aspect ratio, rim diameter, load and speed markings, then calculate sidewall height and diameter using a 225/45 R17 example.
- Staggered vs square wheels: tyres, rotation and AWDCompare square and staggered wheel setups, calculate front-to-rear tyre diameter, and understand rotation limits and all-wheel-drive requirements.
Questions
Do bigger tyres make the speedometer read low?
A larger rolling circumference takes the car further per revolution. If the speedometer was accurate with the original tyres, actual speed will be higher than the indicated speed after fitting larger ones. This calculator shows the change due to size only, not any existing speedometer error.
Is a change within 3% automatically safe?
No. Diameter is only one check. Confirm tyre load and speed ratings, rim approval, brake and body clearance, ABS and stability-control requirements, and the vehicle maker's limits. All-wheel-drive cars can need closely matched rolling circumferences.
Why do the calculated dimensions differ from the tyre maker's?
The size on the sidewall is nominal. Actual dimensions vary with the tyre model, measuring rim, pressure, wear and load. Use the manufacturer's specification for your final decision.
Keep checking
Use these results to plan a setup. Confirm fitment, tyre approval and clearances with a qualified professional before buying or fitting parts.