225/45R18 vs 235/40R18 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 | 225/45R18 | 235/40R18 | Difference |
|---|---|---|---|
| Overall Diameter | 659.7 mm (26") | 645.2 mm (25.4") | -14.5 mm |
| Section Width | 225 mm (8.9") | 235 mm (9.3") | +10 mm |
| Sidewall Height | 101.3 mm | 94 mm | -7.3 mm |
| Circumference | 2072.5 mm | 2027 mm | -45.5 mm |
| Revolutions/Mile | 777 | 794 | +17 |
| Aspect Ratio | 45% | 40% | -5% |
| Rim Diameter | 18" | 18" | 0" |
Key Impact Summary
Speedometer Correction Table
When switching from 225/45R18 to 235/40R18, 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 | 48.9 mph | -1.1 mph |
| 60 mph | 58.7 mph | -1.3 mph |
| 70 mph | 68.5 mph | -1.5 mph |
| 80 mph | 78.2 mph | -1.8 mph |
| 90 mph | 88 mph | -2 mph |
| 100 mph | 97.8 mph | -2.2 mph |
| 110 mph | 107.6 mph | -2.4 mph |
| 120 mph | 117.4 mph | -2.6 mph |
What This Swap Means
Wider tires (235/40R18): 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 7.3mm 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: Decreases by 7.2mm, lowering the center of gravity slightly which can improve handling stability.