Tire & wheel tools
Tire size comparison
Enter the current and the new size to compare width, sidewall, overall diameter, circumference and revs per km — absolute difference and percentage — with the speedometer impact of the diameter change.
Diameter change
+2.98%
New tire is +0.7 in larger in diameter
Swapping 225/45R17 for 245/40R18 changes the diameter by +2.98%.
Old 225/45R17 · ⌀25.0 inNew 245/40R18 · ⌀25.7 in
| Old 225/45R17 | New 245/40R18 | Diff | Diff % | |
|---|---|---|---|---|
| Section width | 8.9 in | 9.6 in | +0.8 in | +8.89% |
| Sidewall height | 4.0 in | 3.9 in | -0.1 in | -3.21% |
| Overall diameter | 25.0 in | 25.7 in | +0.7 in | +2.98% |
| Circumference | 78.5 in | 80.8 in | +2.3 in | +2.98% |
| Revs per km | 501.8 | 487.3 | -14.5 | -2.89% |
| Revs per mile | 807.6 | 784.2 | -23.4 | -2.89% |
Diameter change exceeds ±3%: speedometer error scales the same way and clearance, suspension and TPMS calibration may be affected — verify fitment before buying. Within ±3% is still not a fitment guarantee.
How it is calculated
Each size computes sidewall = width × aspect ÷ 100 and diameter = rim × 25.4 + 2 × sidewall; the table then diffs every metric against the current size as the base.
The diameter row matters most — it scales the speedometer and odometer the same way. ±3% is the common rule of thumb; beyond it, verify fitment, and inside it is still no guarantee.
The overlay draws both tires on one shared ground line to scale; the side-by-side view separates them. The drawings show geometry, not real-world clearance.
How to fill each field
- Current tireThe size on the car now — printed on the sidewall, e.g. 225/45R17.
- New tireThe size you plan to run. Going wider usually means stepping the aspect ratio down to hold the diameter steady.
- Diff and %The diff column is absolute, the percent uses the current tire as the base. The diameter row is the speedometer error; the width row is lateral clearance.
FAQ
How much bigger is 245/40R18 than 225/45R17?
634.3mm vs 653.2mm — the new tire is 18.9mm (0.74in) taller, about +2.98%. At an indicated 60mph the real speed is about 61.8mph.
What diameter change counts as safe?
The common rule of thumb keeps the change within ±3%. Beyond that, speedometer error grows and clearance, suspension and TPMS calibration can be affected — but ±3% is a reference, not a fitment guarantee; the vehicle manual and a real measurement decide.
Why drop the aspect ratio when going wider?
Diameter = rim diameter plus two sidewalls, and the sidewall is width × aspect ratio. A wider tire needs a lower aspect to keep the diameter from growing.
What does a zero diff mean?
Both sizes are geometrically identical — speedometer, circumference and revs are unchanged, as when swapping brands of the same size.
Are the drawn tires real profiles?
No — the circles are scaled from the theoretical diameters and only show the diameter gap against the ground line, not tread shape or shoulders.