Correct indicated speed after tire diameter or axle gear changes — see true speed, error percent, and a full speed map.
Correct indicated speed after tire diameter or axle gear changes.
Tire diameter & indicated speed
Axle ratios
Optional — leave locked when only tires changed.
Instrument cluster · calibration
Indicated vs actual speed
Speedo reads low · +3.1%
Indicated Actual
Speedo reads
60
mph indicated
True speed
61.9
mph actual
Factor
1.0311
Tire Δ
+3.1%
Gear Δ
0%
Speed map
What the road is doing when the needle points at each mark.
Speedo
Actual
Diff
20
20.6
+0.6
30
30.9
+0.9
40
41.2
+1.2
50
51.6
+1.6
60
61.9
+1.9
70
72.2
+2.2
80
82.5
+2.5
90
92.8
+2.8
Speedometer calibration after tire or gear changes
Taller tires make the speedo read low; shorter tires make it read high.
Axle gear swaps change driveshaft speed — include both ratios when the ring-and-pinion changed.
Use the speed map before a road trip or track day so you know what “60” on the needle really means.
Plus-sizing wheels, swapping to a taller all-terrain, or installing steeper gears throws the indicated speed off because the sensor still assumes the stock tire and final drive. This free speedometer calibration calculator combines those two inputs into one correction factor and shows true speed next to the needle reading.
Enter overall diameters (from a tire size calculator or tire maker sheet) and optional axle ratios. The instrument cluster view compares indicated vs actual, and the table maps common speedo marks so you can see error at cruise, highway, and trap-adjacent speeds.
Treat the result as geometry math — ABS thresholds, GPS, and local speed limits still win in the real world. When error is more than a couple percent, plan a programmer, tuner, or speedo-gear fix rather than driving by a knowingly wrong needle.
How to use the speedometer calibration calculator
Enter stock and new tire diameters. Use overall diameter in inches or millimeters — not section width alone.
Set axle ratios if gears changed. Leave both gear fields the same when only tires changed.
Pick an indicated speed. Enter what the needle shows, then read true speed on the gauge and speed map.
Speedometer calibration FAQ
Compare stock and new overall tire diameters (and axle gears if those changed). Actual speed = indicated × (new diameter ÷ stock diameter) × (stock gear ÷ new gear). Use this page to read the error percent and the speed map.
Yes. A taller tire covers more ground per revolution, so the car is going faster than the speedo shows. The actual speed is higher than the indicated speed.
Include both stock and new axle ratios. Numerically higher gears (e.g. 3.73 vs 3.55) spin the speedo sensor faster and usually make the speedo read high unless tires offset it.
Use a tire size calculator for width/aspect/rim → diameter, measure sidewall-to-sidewall loaded height carefully, or read the manufacturer’s published overall diameter.
No. This tool estimates the mechanical/sensor error so you know how far off you are. Correcting the cluster or PCM still needs a tuner, programmer, or speedo gear change depending on the vehicle.