Tire diameter change
+2.98%
Larger diameterThe proposed tire is 18.9 mm taller.
Tire Size Comparison Calculator
Compare two tire sizes on one scale, with explicit diameter, sidewall, circumference, ride-height and speed effects. Wheel offset is outside this tire-only task.
Result updated. 225/45R17 → 245/40R18: nominal diameter changes by +2.98% (larger diameter). The overlay uses the same scale for both tires.
Result
225/45R17 → 245/40R18: nominal diameter changes by +2.98% (larger diameter). The overlay uses the same scale for both tires.
Sidewall change
−3.3 mm
Less sidewallNominal profile-height change, not a comfort or performance rating.
Ride-height change
+9.5 mm
HigherHalf the diameter change, before tire deflection under load.
Estimated actual speed
103.0 km/h
Higher than indicatedAt indicated 100.0 km/h, assuming the original tire was the calibration reference.
Details
- Nominal width change
- +20.0 mm
Swipe to view more, if needed
| Measure | Current | Proposed | Change |
|---|---|---|---|
| Overall diameter | 634.3 mm | 653.2 mm | +18.9 mm |
| Sidewall | 101.3 mm | 98.0 mm | −3.3 mm |
| Circumference | 1992.7 mm | 2052.1 mm | +59.4 mm |
| Revs per mile | 807.6 | 784.2 | −23.4 |
| Revs per km | 501.8 | 487.3 | −14.5 |
Results use nominal geometry and the stated conventions. They are dimensional answers, not a fitment, safety or legality check.
How is tire diameter difference calculated?
Subtract current diameter from proposed diameter. Divide that difference by current diameter and multiply by 100 for the percentage. A positive value means a larger proposed tire, not automatic replacement approval.
Enter the current and proposed complete metric sizes. Changes are proposed minus current, except where the interpretation explicitly describes a derived quantity. A larger rim does not necessarily mean a larger overall tire because the sidewall profile may be smaller.
The diameter percentage uses current diameter as its denominator. Swapping sizes reverses direction but does not necessarily give exactly the negative percentage, because the new reference diameter is different. Identical sizes correctly produce unchanged geometry.
Understand the overlay and changes
Current and proposed tires use a shared scale. The dashed and solid outlines show nominal unloaded diameters and rims, not actual tire shape. Sidewall change is separate from diameter change; width is another independent nominal difference.
Theoretical axle height changes by half the diameter difference. That is a radius comparison, not a prediction of complete suspension ride height, clearance through travel, handling or tire deflection.
Speed effects use the same diameter ratio
If the original tire was the speedometer calibration reference, estimated actual speed = indicated speed × proposed diameter ÷ current diameter. A larger proposed diameter produces higher actual speed at a nonzero indication.
Advanced exposes a reference speed when you want a concrete estimate. Changing output units does not change the tire dimensions. The dedicated Speedometer Error tool emphasizes actual speed and a reference table rather than the full dimensional comparison.
Example: 225/45R17 versus 245/40R18
Inputs: Current tire: 225/45R17; Proposed tire: 245/40R18; Indicated speed: 100 km/h. Metric results below are computed by the same engine as the live tool.
225/45R17 → 245/40R18: nominal diameter changes by +2.98% (larger diameter). The overlay uses the same scale for both tires.
Swipe to view more, if needed
| Measure | Calculated value | Meaning |
|---|---|---|
| Tire diameter change | +2.98% | The proposed tire is 18.9 mm taller. |
| Sidewall change | −3.3 mm | Nominal profile-height change, not a comfort or performance rating. |
| Ride-height change | +9.5 mm | Half the diameter change, before tire deflection under load. |
| Estimated actual speed | 103.0 km/h | At indicated 100.0 km/h, assuming the original tire was the calibration reference. |
Important limitations
Nominal diameter difference is not a universal pass/fail rule. Check vehicle requirements, actual tire dimensions, approved rim widths, load/speed ratings and clearances; this comparison does not approve a replacement.
See the calculation methodology and testing and accuracy for precision, unit conventions and the distinction between nominal geometry and physical measurement.