225/45R17 vs 225/40R18
A calculated comparison of tire dimensions, speedometer change and clearance.
Last checked 12 October 2026Published by Sidewall GuideFormulas follow the Tire and Rim Association and ETRTO size conventions
Within 3%: a common safe change
A similar diameter does not confirm fitment. Check load index, wheel width and clearance.
| Dimension | Current | New | Difference |
|---|---|---|---|
| Overall diameter | 24.97 in (634 mm) | 25.09 in (637 mm) | +0.11 in (+0.5%) |
| Section width | 8.86 in (225 mm) | 8.86 in (225 mm) | 0.00 in (0.0%) |
| Sidewall height | 3.99 in (101 mm) | 3.54 in (90 mm) | −0.44 in (−11.1%) |
| Circumference | 78.45 in (1,993 mm) | 78.81 in (2,002 mm) | +0.36 in (+0.5%) |
| Revolutions per mile | 808 | 804 | −4 (−0.5%) |
Your speedometer
When your speedometer reads 30 / 45 / 60 / 75 mph, you are really doing …
| Speedometer reads | You are really doing |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.2 mph |
| 60 mph | 60.3 mph |
| 75 mph | 75.3 mph |
Your speedometer will read low: you are going faster than it shows.
Your odometer will count 1,000 miles when you have driven 1,005 miles.
Clearance
Ride height: +0.06 in
Tread edge: no change, on the same wheel offset
Ride height is half the diameter change. The width change splits equally to each side of the tire.
Fitment
Can I fit 225/40R18 instead of 225/45R17?
The calculated overall diameter changes by 0.11 in (0.5%). That is within 3%, a common size-change guideline. This calculation cannot confirm fitment for a specific vehicle.
Plus-sizing
Other sizes near 225/45R17
Common sizes on the same wheel and on +1 and +2 wheels whose overall diameter stays within 3% of 225/45R17's 24.97 in.
Same wheel17 in wheel
No common size within 3% on a 17-inch wheel.
+118 in wheel
| Size | Diameter | Difference |
|---|---|---|
| 225/40R18 | 25.09 in | +0.5% |
+219 in wheel
No common size within 3% on a 19-inch wheel.
Next step
Compare your own sizes
Use the calculator with both sizes prefilled, then change either value to match your vehicle or replacement tire.
Compare your own sizes