Speedometer Error: 235/60R18 → 305/40R20

What this means for your vehicle

Changing from 235/60R18 to 305/40R20 shifts overall tire diameter by +1.73%, which produces a proportional 1.73% error in the speedometer and odometer readings. Because 305/40R20 is taller than 235/60R18, each tire revolution covers more ground, so the factory speedometer reads low — you are actually moving faster than the dash shows. At an indicated 30 mph (city driving) your actual speed is 30.5 mph; at 60 mph (highway) it is 61.0 mph; at 75 mph (cruise control) it becomes 76.3 mph. The odometer drifts at the same rate — over 12,000 indicated miles per year, you actually travel 208 miles more than the dash shows, which matters for lease-mileage caps and warranty tracking. Most ECUs are designed to tolerate up to ±3% of calibration drift, so no recalibration is required for legal or safety compliance. If you plan to keep the 305/40R20 setup long-term, an OBD-II programmer or dealer recalibration costs roughly $50–150 and restores accurate readings across the instrument cluster.

Odometer impact

For every indicated mile, you're actually traveling 1.0173 miles. Over 10,000 indicated miles, that's 10173 actual miles. Check your warranty and service intervals accordingly.

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