Speedometer Driven Gear Recalibration Calculator
When changing your vehicle’s wheel and tire size, your speedometer may become inaccurate. To correct this, you may need to replace the speedometer driven gear. Use the calculator below to determine the correct number of teeth for your new driven gear.
How to Use the Speedometer Driven Gear Calculator
Before diving into the details, let’s quickly cover how to use our calculator:
- Enter your original tire diameter in inches (the factory tire size)
- Input your new tire diameter in inches
- Provide your original axle ratio (e.g., 3.73)
- If you’ve changed your axle ratio, enter the new axle ratio
- Input your original driven gear teeth number (can be found on your current gear)
- Optionally, include your speedometer drive gear teeth number
- Click “Calculate” to get the correct number of teeth needed for your new driven gear
The calculator will show you exactly which driven gear you need to restore your speedometer’s accuracy.
Why Speedometer Calibration Matters After Tire Changes
Your vehicle’s speedometer relies on a simple mechanism to calculate speed. In vehicles with mechanical speedometers, a cable connects to the transmission output shaft through a set of gears. The speed reading depends on:
- How fast the wheels rotate
- The diameter of those wheels
- The ratio between the drive and driven gears in your speedometer mechanism
When you change your tire size, you alter a fundamental variable in this equation.
The Impact of Bigger Tires on Speedometer Readings
Larger tires travel a greater distance per rotation than smaller ones. If you upgrade from 28-inch tires to 33-inch tires, each rotation covers approximately 17% more ground. However, your speedometer still counts the same number of rotations, causing it to underreport your actual speed by that same percentage.
This means when your speedometer reads 55 mph, you could actually be traveling at 64 mph or more – a significant discrepancy that can lead to speeding tickets and safety concerns.
Understanding Mechanical Speedometer Calibration
How Mechanical Speedometers Work
A mechanical speedometer consists of two primary gears:
- Drive gear: Fixed to the transmission output and rotates at a speed proportional to the vehicle’s wheels
- Driven gear: Connected to the speedometer cable and drives the gauge needle
The relationship between these gears determines how fast the speedometer needle moves for a given wheel rotation rate.
The Driven Gear: Your Key to Speedometer Recalibration
The driven gear is the component you’ll need to replace when recalibrating your speedometer. These gears come with different numbers of teeth, typically ranging from 16 to 53. The formula for determining the correct driven gear teeth is:
New Driven Gear Teeth = Original Driven Gear Teeth × (Original Tire Diameter ÷ New Tire Diameter) × (New Axle Ratio ÷ Original Axle Ratio)
How to Calculate Speedometer Recalibration
Measuring Your Tire Diameter Accurately
For precise calculations, you need the actual rolling diameter of your tires, not just what’s printed on the sidewall. To measure:
- Inflate your tires to the recommended pressure
- Mark the tire at the bottom where it contacts the ground
- Mark the ground at this same point
- Roll the vehicle forward until the tire makes one complete revolution
- Mark this second point on the ground
- Measure the distance between your ground marks
- Divide by π (3.14159) to get the diameter
Locating Your Speedometer Driven Gear
In most vehicles, the driven gear is located where the speedometer cable connects to the transmission. Typically, you’ll need to:
- Locate the speedometer cable connection at the transmission
- Remove any housing or protective covers
- Extract the current driven gear
- Note the number of teeth on the current gear
Installing the New Driven Gear
Once you’ve calculated the correct number of teeth:
- Purchase the appropriate replacement gear from your vehicle manufacturer or aftermarket supplier
- Remove the old driven gear
- Install the new driven gear
- Reassemble any housing or protective covers
- Test drive your vehicle to confirm improved accuracy
Axle Ratio Changes and Speedometer Calibration
If you’ve also changed your vehicle’s axle ratio, this further affects your speedometer reading. A lower numerical axle ratio (e.g., changing from 3.73 to 3.08) means fewer engine revolutions per wheel rotation, which also slows down your speedometer.
The calculator accounts for this by multiplying by the ratio of new to old axle ratios.
FAQ: Speedometer Calibration Questions
Q. How do you calibrate a speedometer after changing tire size?
For mechanical speedometers, you need to replace the driven gear with one that has the correct number of teeth. For electronic speedometers, you might need a calibration device or to update the vehicle’s ECU with the new tire specifications.
Q. Why is my speedometer wrong after a tire change?
Your speedometer calculates speed based on wheel rotations. Larger tires travel farther per rotation, making your speedometer read slower than your actual speed. Smaller tires have the opposite effect.
Q. Do bigger tires affect speedometer?
Yes, bigger tires make your speedometer read slower than your actual speed. As a rule of thumb, for every 3% increase in tire diameter, your speedometer will be approximately 3% slower.
Q. How much does tire size affect speedometer?
The formula is: % Error = [(New Diameter ÷ Original Diameter) – 1] × 100. For example, going from 28″ to 33″ tires creates about a 17.9% error, meaning when your speedometer shows 50 mph, you’re actually traveling at 58.9 mph.
Q. What is a driven gear in a speedometer?
The driven gear is the gear attached to your speedometer cable at the transmission. It works in conjunction with the drive gear to transfer rotation from your transmission to your speedometer at the correct ratio.
Q. Can I recalibrate my mechanical speedometer?
Yes, you can recalibrate a mechanical speedometer by replacing the driven gear with one that has the appropriate number of teeth for your new tire size and/or axle ratio.
Conclusion
Maintaining an accurate speedometer isn’t just about avoiding speeding tickets—it’s about safety and proper vehicle maintenance. When you change your tire size or axle ratio, taking the time to recalibrate your speedometer ensures:
- You know your actual speed while driving
- Your odometer accurately tracks vehicle mileage
- Your vehicle’s computer systems receive correct speed data
Our Speedometer Driven Gear Recalibration Calculator makes this process straightforward, helping you find the exact gear you need. Remember that in some jurisdictions, having an accurate speedometer is a legal requirement, making this adjustment not just practical but potentially mandatory.
