Speedometer Correction Calculator

Speedometer Correction is evaluated from Original Tire Diameter, New Tire Diameter and Indicated Speed. The calculation reports Actual Speed, Speedometer Error and Odometer Correction Factor.

Results

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About the Speedometer Correction Calculator

Speedometer Correction is treated here as a quantitative relation between Original Tire Diameter, New Tire Diameter and Indicated Speed and Actual Speed, Speedometer Error, Odometer Correction Factor and Actual Miles per 100 Indicated Miles.

The calculator uses a multi formula configuration. Each reported value is read as a direct evaluation of the stored rules with the declared field formats and units.

Formula basis:
Actual speed = indicated speed x (new_tire_diameter / original_tire_diameter)
Speedometer error (%) = (new_diameter / original_diameter - 1) x 100
Odometer factor = new_diameter / original_diameter
True miles per 100 indicated = odometer_factor x 100

Interpret the outputs in the order shown by the result fields. Optional inputs affect only the outputs that depend on those variables.

Formula & How It Works

The calculation applies the following relations exactly as recorded in the metadata:

Actual speed = indicated speed x (new_tire_diameter / original_tire_diameter)
Speedometer error (%) = (new_diameter / original_diameter - 1) x 100
Odometer factor = new_diameter / original_diameter
True miles per 100 indicated = odometer_factor x 100

Each output field is produced by substituting the supplied inputs into the relevant relation and then applying the declared rounding or text format.

Worked Examples

Example 1: Jeep Wrangler: stock 30" tires upgraded to 33" tires, speedometer shows 65 mph

Inputs

orig_diam: 30 new_diam: 33 indicated_speed: 65
Actual Speed: 71.5 mph. Speedometer Error: 10%. Odometer Correction Factor: 1.1 x. Actual Miles per 100 Indicated Miles: 110 miles

With Original Tire Diameter = 30, New Tire Diameter = 33 and Indicated Speed = 65 as the stated inputs, the result is Actual Speed = 71.5 mph, Speedometer Error = 10% and Odometer Correction Factor = 1.1 x. Each value corresponds to the declared output fields.

Example 2: Chevy Silverado: factory 31.6" tires swapped to 35" for off-road, speedometer shows 70 mph

Inputs

orig_diam: 31.6 new_diam: 35 indicated_speed: 70
Actual Speed: 77.5 mph. Speedometer Error: 10.76%. Odometer Correction Factor: 1.1076 x. Actual Miles per 100 Indicated Miles: 110.8 miles

With Original Tire Diameter = 31.6, New Tire Diameter = 35 and Indicated Speed = 70 as the stated inputs, the result is Actual Speed = 77.5 mph, Speedometer Error = 10.76% and Odometer Correction Factor = 1.1076 x. Each value corresponds to the declared output fields.

Example 3: Daily driver downsize: stock 235/55R18 (28.2") swapped to 225/50R17 (26.9") budget tires, indicated 60 mph

Inputs

orig_diam: 28.2 new_diam: 26.9 indicated_speed: 60
Actual Speed: 57.2 mph. Speedometer Error: -4.61%. Odometer Correction Factor: 0.9539 x. Actual Miles per 100 Indicated Miles: 95.4 miles

With Original Tire Diameter = 28.2, New Tire Diameter = 26.9 and Indicated Speed = 60 as the stated inputs, the result is Actual Speed = 57.2 mph, Speedometer Error = -4.61% and Odometer Correction Factor = 0.9539 x. Each value corresponds to the declared output fields.

Example 4: BMW M3 track build: OEM 275/35R19 (26.6") replaced with 305/30R19 (25.8") Michelin PS Cup2, GPS shows 130 mph at track

Inputs

orig_diam: 26.6 new_diam: 25.8 indicated_speed: 130
Actual Speed: 126.1 mph. Speedometer Error: -3.01%. Odometer Correction Factor: 0.9699 x. Actual Miles per 100 Indicated Miles: 97 miles

With Original Tire Diameter = 26.6, New Tire Diameter = 25.8 and Indicated Speed = 130 as the stated inputs, the result is Actual Speed = 126.1 mph, Speedometer Error = -3.01% and Odometer Correction Factor = 0.9699 x. Each value corresponds to the declared output fields.

Common Use Cases

  • Correct speedometer reading after changing to larger tires
  • Calculate actual speed when speedometer reads a given value
  • Determine odometer error from non-stock tire sizes