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# GM Speedometer Gear Calculator Online

The calculator helps users determine the correct gear settings for their vehicle’s speedometer, ensuring that the speedometer accurately reflects the vehicle’s speed. This accuracy is vital for complying with speed limits, avoiding speeding tickets, and maintaining safe driving conditions.

## Formula of GM Speedometer Gear Calculator

To achieve accurate results, the calculator uses specific formulas:

• Gear Ratio = (Driven Gear Teeth) / (Drive Gear Teeth) Where:
• Drive Gear Teeth refers to the number of teeth on the drive gear connected to the transmission output shaft.
• Driven Gear Teeth refers to the number of teeth on the driven gear connected to the speedometer cable. It is calculated as follows:
• Driven Teeth = (Drive Teeth * Axle Ratio * Tire Rev. per Mile) / 1001
• To complete these calculations, you also need the Tire Rev. per Mile, calculated by:
• Tire Rev. per Mile = 20168 / Tire Diameter (inches)

## Example of GM Speedometer Gear Calculator

Let’s walk through a practical example using the GM Speedometer Gear Calculator. Assume you own a GM vehicle and decide to upgrade your tires from 26 inches to 28 inches in diameter while maintaining the same axle ratio and drive gear teeth count.

Step-by-step Calculation:

1. Original Setup:
• Tire Diameter: 26 inches
• Axle Ratio: 3.5
• Drive Gear Teeth: 20
• According to our table, the original Driven Gear Teeth would be 36.
2. New Setup:
• New Tire Diameter: 28 inches
• Axle Ratio (unchanged): 3.5
• Drive Gear Teeth (unchanged): 20
• From the table, the new Driven Gear Teeth should be adjusted to 34.

Calculation Details:

• The change in tire diameter affects the tire revolutions per mile. According to the formula:
• Tire Rev. per Mile = 20168 / Tire Diameter (inches)
• Original Tire Rev. per Mile = 20168 / 26 ≈ 776 revolutions per mile
• New Tire Rev. per Mile = 20168 / 28 ≈ 720 revolutions per mile
• Using the formula for Driven Gear Teeth:
• Driven Teeth = (Drive Teeth * Axle Ratio * Tire Rev. per Mile) / 1001
• Original Driven Teeth = (20 * 3.5 * 776) / 1001 ≈ 36
• New Driven Teeth = (20 * 3.5 * 720) / 1001 ≈ 34

Conclusion: With the tire diameter change from 26 to 28 inches, you would need to change the driven gear from 36 teeth to 34 teeth to maintain accurate speedometer readings. This example demonstrates how changing one variable (in this case, the tire diameter) affects the overall gear settings necessary for accurate vehicle performance metrics.