Speedometer calculator

True speed = indicated speed × new overall diameter / old overall diameter. The calculation uses the old tire diameter as its baseline. These are unloaded nominal dimensions; real tires measure slightly differently.

For 265/70R17 → 285/70R17, nominal diameter increases from 31.6 in (803 mm) to 32.7 in (831 mm). Using the unrounded diameters gives a multiplier of 1.0349, so 60 mph indicated becomes 62.1 mph true by the formula. Use the tire size comparison to review diameter and width changes together.

Read each row as indicated speed followed by calculated true speed. For the nominal 265/70R17 → 285/70R17 change, 30 mph indicated becomes 31.0 mph true, while 80 mph becomes 82.8 mph. At 100 km/h indicated, true speed is 103.5 km/h by the formula.

The percentage stays constant across the table, but the difference in mph grows with speed. For the second example, 275/60R20 → 275/65R20, nominal diameter increases from 33.0 in (838 mm) to 34.1 in (866 mm). Its +3.3% correction gives 62.0 mph true at 60 mph indicated, or 103.3 km/h true at 100 km/h indicated.

Discount Tire says taller tires make the speedometer read slower than actual speed, while shorter tires make it read faster. The calculator shows the tire-diameter effect, not a measurement of the instrument’s existing accuracy.

49 CFR 393.82 applies to buses, trucks and truck-tractors under FMCSA rules. It requires speedometer accuracy within ±5 mph at 50 mph. It also requires a speedometer indicating speed in miles per hour and/or kilometers per hour.

No passenger-car accuracy rule was found in FMVSS. This regulatory check is dated October 15, 2026. The federal commercial-vehicle tolerance should not be presented as a passenger-car accuracy allowance.

The ±3% diameter window is a tire-shop rule of thumb, not a law or a vehicle maker specification. Both nominal changes shown here fall outside that window. Fit depends on the vehicle, wheels and suspension.

The odometer uses the same diameter correction factor as the speedometer. Actual distance equals indicated distance multiplied by the new-to-old diameter ratio. For the nominal 265/70R17 → 285/70R17 example, actual distance is +3.5% above the indicated distance.

Discount Tire recommends recalibrating the speedometer whenever tire size changes and directs customers to a professional auto shop. Ask a dealer or professional auto shop about calibration and whether a programming tool is appropriate for the vehicle. Calibration is separate from calculating the nominal tire-size correction.