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Speedometer Gear Calculator

Understanding Speedometer Gear Calculation

The speedometer gear calculation involves determining the appropriate size of the driven gear based on the tire diameter, axle ratio, and the number of teeth on the drive gear.

Formulas Used

Revolutions Per Mile:

$R = \frac{63360}{\pi \times D}$

Number of Teeth on Driven Gear:

$T_d = \frac{A \times R \times T_g}{1001}$

where:

$R$ = Revolutions per mile
$D$ = Tire diameter (inches)
$T_d$ = Number of teeth on driven gear
$A$ = Axle ratio
$T_g$ = Number of teeth on drive gear

Calculation Example

Suppose the tire diameter is 20 inches, the axle ratio is 3, and the number of teeth on the drive gear is 40:

Revolutions Per Mile:

$R = \frac{63360}{\pi \times 20} \approx 1008.4$

Number of Teeth on Driven Gear:

$T_d = \frac{3 \times 1008.4 \times 40}{1001} \approx 121$

Therefore, you would need a driven gear with 121 teeth for a tire size of 20 inches.