225/45R17 vs 225/40R18 Side-by-side tire size comparison with specs and compatibility
Compatibility
+0.46% diameter difference
✓ Safe Swap
This tire swap is within the safe 3% diameter threshold. It should not cause issues with speedometer accuracy, ABS, or traction control systems.
Full Specifications Comparison
| Specification | 225/45R17 | 225/40R18 | Difference |
|---|---|---|---|
| Overall Diameter | 634.3 mm (25") | 637.2 mm (25.1") | +2.9 mm |
| Section Width | 225 mm (8.9") | 225 mm (8.9") | 0 mm |
| Sidewall Height | 101.3 mm | 90 mm | -11.3 mm |
| Circumference | 1992.7 mm | 2001.8 mm | +9.1 mm |
| Revolutions/Mile | 808 | 804 | -4 |
| Aspect Ratio | 45% | 40% | -5% |
| Rim Diameter | 17" | 18" | +1" |
Key Impact Summary
Ground Clearance
+1.5 mm
Speedometer Error
+0.46%
Width Change
0 mm
Sidewall Change
-11.3 mm
Speedometer Correction Table
When switching from 225/45R17 to 225/40R18, your speedometer will under-read. Here is the correction at common speeds:
| Speedometer Shows | Actual Speed (mph) | Difference |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 mph |
| 40 mph | 40.2 mph | +0.2 mph |
| 50 mph | 50.2 mph | +0.2 mph |
| 60 mph | 60.3 mph | +0.3 mph |
| 70 mph | 70.3 mph | +0.3 mph |
| 80 mph | 80.4 mph | +0.4 mph |
| 90 mph | 90.4 mph | +0.4 mph |
| 100 mph | 100.5 mph | +0.5 mph |
| 110 mph | 110.5 mph | +0.5 mph |
| 120 mph | 120.5 mph | +0.5 mph |
What This Swap Means
Shorter sidewall: The 11.3mm decrease in sidewall height improves steering response and cornering precision. However, ride quality becomes firmer and the wheels are more vulnerable to pothole damage. This is a common trade-off when upsizing wheels (plus-sizing).
Ground clearance: Increases by 1.5mm, providing slightly better approach angles for off-road driving and speed bump clearance.
Vehicles Using 225/45R17
Vehicles Using 225/40R18
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Individual Tire Specs
Frequently Asked Questions
Yes, this swap is generally safe. The diameter difference is +0.46% (+2.9mm). This is within the safe 3% threshold and should not significantly affect your speedometer, ABS, or traction control.
Switching from 225/45R17 to 225/40R18 creates a +0.46% speedometer error. Your actual speed will be faster than displayed — at an indicated 60 mph, you'll actually be going 60.3 mph.
Switching from 225/45R17 to 225/40R18 changes ground clearance by +1.5mm. The vehicle will sit 1.5mm higher. Ground clearance change equals half the overall diameter difference.