205/45R17 vs 225/40R18 Side-by-side tire size comparison with specs and compatibility
This tire swap is between 3-5% diameter difference. Check for fender clearance and be aware of speedometer error. May require recalibration.
Full Specifications Comparison
| Specification | 205/45R17 | 225/40R18 | Difference |
|---|---|---|---|
| Overall Diameter | 616.3 mm (24.3") | 637.2 mm (25.1") | +20.9 mm |
| Section Width | 205 mm (8.1") | 225 mm (8.9") | +20 mm |
| Sidewall Height | 92.3 mm | 90 mm | -2.3 mm |
| Circumference | 1936.2 mm | 2001.8 mm | +65.6 mm |
| Revolutions/Mile | 831 | 804 | -27 |
| Aspect Ratio | 45% | 40% | -5% |
| Rim Diameter | 17" | 18" | +1" |
Key Impact Summary
Speedometer Correction Table
When switching from 205/45R17 to 225/40R18, your speedometer will under-read. Here is the correction at common speeds:
| Speedometer Shows | Actual Speed (mph) | Difference |
|---|---|---|
| 30 mph | 31 mph | +1 mph |
| 40 mph | 41.4 mph | +1.4 mph |
| 50 mph | 51.7 mph | +1.7 mph |
| 60 mph | 62 mph | +2 mph |
| 70 mph | 72.4 mph | +2.4 mph |
| 80 mph | 82.7 mph | +2.7 mph |
| 90 mph | 93.1 mph | +3.1 mph |
| 100 mph | 103.4 mph | +3.4 mph |
| 110 mph | 113.7 mph | +3.7 mph |
| 120 mph | 124.1 mph | +4.1 mph |
What This Swap Means
Wider tires (225/40R18): The 20mm wider contact patch provides better grip in dry conditions and improved cornering stability. However, wider tires increase rolling resistance slightly, which may affect fuel economy by 1-2%. They are also more susceptible to hydroplaning in wet conditions.
Shorter sidewall: The 2.3mm 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 10.5mm, providing slightly better approach angles for off-road driving and speed bump clearance.