235/45R18 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 | 235/45R18 | 245/40R18 | Difference |
|---|---|---|---|
| Overall Diameter | 668.7 mm (26.3") | 653.2 mm (25.7") | -15.5 mm |
| Section Width | 235 mm (9.3") | 245 mm (9.6") | +10 mm |
| Sidewall Height | 105.8 mm | 98 mm | -7.8 mm |
| Circumference | 2100.8 mm | 2052.1 mm | -48.7 mm |
| Revolutions/Mile | 766 | 784 | +18 |
| Aspect Ratio | 45% | 40% | -5% |
| Rim Diameter | 18" | 18" | 0" |
Key Impact Summary
Speedometer Correction Table
When switching from 235/45R18 to 245/40R18, your speedometer will over-read. Here is the correction at common speeds:
| Speedometer Shows | Actual Speed (mph) | Difference |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 mph |
| 40 mph | 39.1 mph | -0.9 mph |
| 50 mph | 48.8 mph | -1.2 mph |
| 60 mph | 58.6 mph | -1.4 mph |
| 70 mph | 68.4 mph | -1.6 mph |
| 80 mph | 78.1 mph | -1.9 mph |
| 90 mph | 87.9 mph | -2.1 mph |
| 100 mph | 97.7 mph | -2.3 mph |
| 110 mph | 107.5 mph | -2.5 mph |
| 120 mph | 117.2 mph | -2.8 mph |
What This Swap Means
Wider tires (245/40R18): The 10mm 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 7.8mm 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: Decreases by 7.7mm, lowering the center of gravity slightly which can improve handling stability.