Speedometer Error: 245/35ZR20 → 255/35R21

What this means for your vehicle

Changing from 245/35ZR20 to 255/35R21 shifts overall tire diameter by +4.77%, which produces a proportional 4.77% error in the speedometer and odometer readings. Because 255/35R21 is taller than 245/35ZR20, 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 31.4 mph; at 60 mph (highway) it is 62.9 mph; at 75 mph (cruise control) it becomes 78.6 mph. The odometer drifts at the same rate — over 12,000 indicated miles per year, you actually travel 572 miles more than the dash shows, which matters for lease-mileage caps and warranty tracking. At 3–5% error, recalibration becomes worthwhile — some OBD-II tuners can adjust the signal, and most ECUs still accept it, but ABS and TPMS become less reliable. If you plan to keep the 255/35R21 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 1.0477 miles. Over 10,000 indicated miles, that's 10477 actual miles. Check your warranty and service intervals accordingly.

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