255/45R20 vs 275/40R21 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 | 255/45R20 | 275/40R21 | Difference |
|---|---|---|---|
| Overall Diameter | 737.5 mm (29") | 753.4 mm (29.7") | +15.9 mm |
| Section Width | 255 mm (10") | 275 mm (10.8") | +20 mm |
| Sidewall Height | 114.8 mm | 110 mm | -4.8 mm |
| Circumference | 2316.9 mm | 2366.9 mm | +50 mm |
| Revolutions/Mile | 695 | 680 | -15 |
| Aspect Ratio | 45% | 40% | -5% |
| Rim Diameter | 20" | 21" | +1" |
Key Impact Summary
Speedometer Correction Table
When switching from 255/45R20 to 275/40R21, your speedometer will under-read. Here is the correction at common speeds:
| Speedometer Shows | Actual Speed (mph) | Difference |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 mph |
| 40 mph | 40.9 mph | +0.9 mph |
| 50 mph | 51.1 mph | +1.1 mph |
| 60 mph | 61.3 mph | +1.3 mph |
| 70 mph | 71.5 mph | +1.5 mph |
| 80 mph | 81.7 mph | +1.7 mph |
| 90 mph | 91.9 mph | +1.9 mph |
| 100 mph | 102.2 mph | +2.2 mph |
| 110 mph | 112.4 mph | +2.4 mph |
| 120 mph | 122.6 mph | +2.6 mph |
What This Swap Means
Wider tires (275/40R21): 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 4.8mm 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: Increases by 7.9mm, providing slightly better approach angles for off-road driving and speed bump clearance.