Speedometer Gear Calculator

Enter your drive gear teeth, axle ratio, and tire dimensions to calculate the correct speedometer driven gear for your vehicle. The Speedometer Gear Calculator computes tire revolutions per mile, the ideal driven gear tooth count, and your estimated speedometer error — so you can pick the right gear and keep your speedo reading true.

Number of teeth on the drive gear mounted to the transmission output shaft.

Your rear axle gear ratio (e.g. 3.73, 4.10). Found on axle tag or door sticker.

Select your transmission to check if the driven gear count falls in the valid range.

Enter your current driven gear tooth count to see how far off your speedometer is.

mm

First number on your tire sidewall (e.g. 245 in 245/45R17). Leave tire fields blank if entering diameter directly.

%

Middle number on your tire sidewall (e.g. 45 in 245/45R17).

in

Last number on your tire sidewall (e.g. 17 in 245/45R17).

in

Enter overall tire diameter directly, or fill in the P-metric fields above to calculate it.

Results

Suggested Driven Gear (nearest whole tooth)

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Calculated Tire Diameter

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Tire Revolutions per Mile

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Ideal Driven Gear (exact)

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Est. Speedometer Error (with suggested gear)

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Est. Speedometer Error (with current gear)

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Driven Gear Comparison: Ideal vs. Suggested vs. Current

Frequently Asked Questions

My transmission is electronic (VSS). Will this calculator work for me?

No — electronic transmissions like the 4L60E (1993+), 4L80E, 6L80/90, and most modern Ford and Chrysler units use a Vehicle Speed Sensor (VSS) rather than mechanical gears. To correct the speedometer reading on those, you need a tune, a programmer, or a signal adapter — not a physical gear swap.

Where do I find my drive gear tooth count?

The drive gear sits on the transmission output shaft and is visible once you remove the speedometer cable housing. You can count the teeth directly, or check the transmission manufacturer's documentation. Common GM TH350 drive gears are 7 or 8 teeth; TH400 units typically use 15 or 16 teeth.

Should I round up or round down on the driven gear tooth count?

It depends which direction you want any residual error to fall. Rounding up gives a reading slightly below actual speed (speedo reads low), while rounding down causes the speedo to read slightly high. An error within ±2% is generally considered excellent; 2–5% is acceptable; above 5% warrants attention.

What driven gear housings are available for GM transmissions?

GM TH350 and TH400 use a single-clip square housing. The 700R4/4L60 (gear-driven versions) use a two-clip housing. Driven gears are color-coded by tooth count, so always verify the tooth count printed or stamped on the gear rather than relying solely on color, as manufacturers sometimes vary.

My speedometer is still off after changing the gear. What else affects it?

Several factors can cause residual error: tire pressure and load affect actual rolling diameter, aftermarket wheels with different offsets, worn tires that are smaller than their rated diameter, or a speedometer head that is out of calibration. A GPS-based speedometer app is a good way to verify true ground speed.

How does tire size affect speedometer accuracy?

The speedometer gear system is calibrated around a specific tire diameter. A taller tire travels farther per revolution, so the speedo reads low. A shorter tire reads high. This calculator uses your tire's actual (or P-metric-derived) diameter to determine the correct gear, compensating for any tire size change.

How do I convert a P-metric tire size to overall diameter?

The formula is: Diameter (in) = Wheel Diameter + 2 × (Tire Width mm × Aspect Ratio% / 100 / 25.4). For example, a 245/45R17 tire calculates to 17 + 2 × (245 × 0.45 / 25.4) ≈ 25.69 inches. This calculator does that conversion automatically when you fill in the P-metric fields.

What is the formula used to calculate the ideal driven gear teeth?

Ideal Driven Gear = (Tire Revolutions per Mile × Drive Gear Teeth) / (Axle Ratio × 1001). Tire revolutions per mile equals 20,168 / Tire Diameter (in). The 1001 constant is a standard transmission output factor used in most mechanical speedometer gear calculations.

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