How do I calculate the correct speedometer gear for my vehicle?

To calculate the correct speedometer driven gear, you need three pieces of information: your axle ratio (found on the differential tag or by counting ring/pinion teeth), your tire diameter or revolutions per mile, and your transmission's drive gear tooth count. The formula is: Driven Gear = (Axle Ratio × Tire Rev/Mile × Drive Gear) / 1001. Our calculator automates this process.

What causes speedometer error after changing tires or gears?

Speedometer error occurs when the relationship between tire rotation and speedometer calibration changes. Larger tires rotate fewer times per mile, causing the speedometer to read slower than actual speed. Changing axle ratios (gear swaps) also affects this relationship. The speedometer gear must be recalculated to correct the error.

What's the difference between mechanical and electronic speedometers?

Mechanical speedometers use a cable connected to a gear in the transmission/transfer case. The driven gear teeth count determines calibration. Electronic speedometers use a Vehicle Speed Sensor (VSS) that generates electrical pulses. Calibration is done through PCM programming or an aftermarket calibrator based on pulses per mile.

How do I find my transmission's drive gear tooth count?

The drive gear is the gear on the transmission output shaft that meshes with the driven gear. Common counts: GM TH350/TH400/4L60 = 15 teeth, Ford C4/C6/AOD = 8 teeth, Chrysler A727/A518 = 8 teeth. For manuals: GM Muncie = 19 teeth, Ford Toploader = 7 teeth. Check our transmission database for your specific transmission.

What if my calculated gear isn't available?

Speedometer gears typically come in 1-tooth increments within a specific range. If your calculation falls between available teeth (e.g., 38.5), choose the nearest whole number gear. A slight error (1-2%) is usually acceptable. For more precision, consider an aftermarket speedometer recalibrator or GPS speedometer.

Can I use my old speedometer gear with new tires?

You can continue using your current gear, but your speedometer will read incorrectly. For example, switching from 30" to 33" tires (10% larger) will cause your speedometer to read about 10% slower than actual speed. At an indicated 60 mph, you'd actually be traveling about 66 mph.

How do I calibrate an electronic speedometer?

Electronic speedometers require PCM reprogramming or an aftermarket calibrator. Calculate your vehicle's pulses per mile: Axle Ratio × Tire Rev/Mile × Pulses Per Revolution. Compare this to the factory setting and adjust accordingly. Devices like Dakota Digital, Speedcal, or JET Performance units allow field calibration.

Does changing my axle ratio affect the speedometer?

Yes, significantly. If you swap from 3.08 to 3.73 gears (21% numerically higher), your speedometer will read about 21% faster than actual speed. You'll need a higher tooth count driven gear or electronic recalibration to correct it.

What's the speedometer gear range for my transmission?

Each transmission has a specific driven gear range based on its housing design. GM TH350/TH400/4L60: 34-45 teeth. Ford C4/C6/AOD: 16-23 teeth. Chrysler A727/A518: 21-39 teeth. Using a gear outside this range may not mesh properly or fit the housing.

How accurate should my speedometer be?

Most manufacturers calibrate speedometers to read slightly high (2-5%) for safety and legal reasons. After modification, aim for less than 3% error. Federal regulations allow up to 5% error at 50 mph. For racing or precision applications, use a GPS speedometer for verification.

How do I calculate the correct speedometer gear for my vehicle?

To calculate the correct speedometer driven gear, you need three pieces of information: your axle ratio (found on the differential tag or by counting ring/pinion teeth), your tire diameter or revolutions per mile, and your transmission's drive gear tooth count. The formula is: Driven Gear = (Axle Ratio × Tire Rev/Mile × Drive Gear) / 1001. Our calculator automates this process.

What causes speedometer error after changing tires or gears?

Speedometer error occurs when the relationship between tire rotation and speedometer calibration changes. Larger tires rotate fewer times per mile, causing the speedometer to read slower than actual speed. Changing axle ratios (gear swaps) also affects this relationship. The speedometer gear must be recalculated to correct the error.

What's the difference between mechanical and electronic speedometers?

Mechanical speedometers use a cable connected to a gear in the transmission/transfer case. The driven gear teeth count determines calibration. Electronic speedometers use a Vehicle Speed Sensor (VSS) that generates electrical pulses. Calibration is done through PCM programming or an aftermarket calibrator based on pulses per mile.

How do I find my transmission's drive gear tooth count?

The drive gear is the gear on the transmission output shaft that meshes with the driven gear. Common counts: GM TH350/TH400/4L60 = 15 teeth, Ford C4/C6/AOD = 8 teeth, Chrysler A727/A518 = 8 teeth. For manuals: GM Muncie = 19 teeth, Ford Toploader = 7 teeth. Check our transmission database for your specific transmission.

What if my calculated gear isn't available?

Speedometer gears typically come in 1-tooth increments within a specific range. If your calculation falls between available teeth (e.g., 38.5), choose the nearest whole number gear. A slight error (1-2%) is usually acceptable. For more precision, consider an aftermarket speedometer recalibrator or GPS speedometer.

Can I use my old speedometer gear with new tires?

You can continue using your current gear, but your speedometer will read incorrectly. For example, switching from 30" to 33" tires (10% larger) will cause your speedometer to read about 10% slower than actual speed. At an indicated 60 mph, you'd actually be traveling about 66 mph.

How do I calibrate an electronic speedometer?

Electronic speedometers require PCM reprogramming or an aftermarket calibrator. Calculate your vehicle's pulses per mile: Axle Ratio × Tire Rev/Mile × Pulses Per Revolution. Compare this to the factory setting and adjust accordingly. Devices like Dakota Digital, Speedcal, or JET Performance units allow field calibration.

Does changing my axle ratio affect the speedometer?

Yes, significantly. If you swap from 3.08 to 3.73 gears (21% numerically higher), your speedometer will read about 21% faster than actual speed. You'll need a higher tooth count driven gear or electronic recalibration to correct it.

What's the speedometer gear range for my transmission?

Each transmission has a specific driven gear range based on its housing design. GM TH350/TH400/4L60: 34-45 teeth. Ford C4/C6/AOD: 16-23 teeth. Chrysler A727/A518: 21-39 teeth. Using a gear outside this range may not mesh properly or fit the housing.

How accurate should my speedometer be?

Most manufacturers calibrate speedometers to read slightly high (2-5%) for safety and legal reasons. After modification, aim for less than 3% error. Federal regulations allow up to 5% error at 50 mph. For racing or precision applications, use a GPS speedometer for verification.

Speedometer Gear Calculator

Professional speedometer calibration calculator for mechanical and electronic speedometers. Calculate the correct driven gear teeth or electronic calibration after tire and axle ratio changes.

Instant Calculations30+ Transmissions24 Tire SizesMetric & Imperial

Speedometer Type

Select Transmission (Optional)

Select Tire Size (Optional)

Axle Ratio

e.g., 3.55, 3.73, 4.10

Tire Diameter (inches)

638 rev/mile

Drive Gear Teeth

On transmission output

Formula Reference

Mechanical Speedometer

Driven Gear = (Axle × Rev/Mile × Drive) / 1001

Rev/Mile = 20168 / Tire Diameter (inches)

Electronic Speedometer

PPM = Axle Ratio × Rev/Mile × Pulses/Rev

Match PPM to factory calibration value

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What Users Say

4.9

Based on 1,250 reviews

“This calculator saved me hours of trial and error. Changed my axle ratio to 4.10s with 33" tires, calculated the exact driven gear, and speedometer reads perfectly!”

Jason M.

Jeep Owner

“Running a TH400 with 4.56 gears and 35s. This tool nailed the 44-tooth gear recommendation. No more guessing at the parts store.”

Mike R.

Off-Road Enthusiast

“Great resource for my restoration shop. The transmission database with drive gear specs is incredibly useful for quick lookups.”

Chris B.

Shop Owner

“Used this for my LS swap project with a T56. The electronic speedometer calibration info was exactly what I needed.”

Dave K.

LS Swap Builder

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