225/45R17 vs 245/40R18 Side-by-side tire size comparison with specs and compatibility
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 | 245/40R18 | Difference |
|---|---|---|---|
| Overall Diameter | 634.3 mm (25") | 653.2 mm (25.7") | +18.9 mm |
| Section Width | 225 mm (8.9") | 245 mm (9.6") | +20 mm |
| Sidewall Height | 101.3 mm | 98 mm | -3.3 mm |
| Circumference | 1992.7 mm | 2052.1 mm | +59.4 mm |
| Revolutions/Mile | 808 | 784 | -24 |
| Aspect Ratio | 45% | 40% | -5% |
| Rim Diameter | 17" | 18" | +1" |
Key Impact Summary
Speedometer Correction Table
When switching from 225/45R17 to 245/40R18, your speedometer will under-read. Here is the correction at common speeds:
| Speedometer Shows | Actual Speed (mph) | Difference |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 mph |
| 40 mph | 41.2 mph | +1.2 mph |
| 50 mph | 51.5 mph | +1.5 mph |
| 60 mph | 61.8 mph | +1.8 mph |
| 70 mph | 72.1 mph | +2.1 mph |
| 80 mph | 82.4 mph | +2.4 mph |
| 90 mph | 92.7 mph | +2.7 mph |
| 100 mph | 103 mph | +3 mph |
| 110 mph | 113.3 mph | +3.3 mph |
| 120 mph | 123.6 mph | +3.6 mph |
What This Swap Means
Wider tires (245/40R18): 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 3.3mm 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 9.5mm, providing slightly better approach angles for off-road driving and speed bump clearance.