Speedometer Gear Calculator - Mechanical Drive and Driven Gear Sizing

A speedometer gear calculator sizes the gears inside a cable-driven speedometer so the needle matches the actual road speed. It takes tire diameter, axle ratio, and drive-gear teeth, and returns the driven-gear teeth needed for a correct reading.

• • Restoring a classic vehicle: Pick the right driven gear when the original part is missing or mis-matched after a transmission swap.
• • Replacing bigger or smaller tires: Rescale the speedometer hardware whenever the tire diameter changes - the most common cause of an inaccurate reading.
• • Changing the axle ratio: Re-spec the driven gear after installing a different ring-and-pinion so the speedometer stays accurate at highway speeds.
• • Cross-checking an OBD-II app: Verify the gear math an OBD-II app uses against the real cable-driven hardware on older trucks.

A mechanical speedometer is one of the oldest driver-information devices. A flexible cable runs from the transmission output to the gauge, and the rotation count of that cable - not the wheel directly - drives the needle. The calculator closes the loop between wheel rotation and cable rotation.

If any variable changes, the cable turns at the wrong rate and the gauge reads fast or slow. Bigger tires read low because the wheel covers more ground per turn. A different axle ratio changes the wheel-to-driveshaft ratio, which also skews the reading.

If you only need to know how far off the needle is on the current setup, our Speedometer Calibration Calculator reads the indicated speed against the actual road speed for any tire change.

The speedometer gear calculator runs the tire-diameter, axle-ratio, and drive-gear-tooth inputs through the standard mechanical-speedometer gearing relationship. Intermediate results are shown so you can see why the answer changes when you change a tire.

• Tire diameter (D): Outside diameter of the mounted tire in inches. The calculator converts it to revolutions per mile internally.
• Axle ratio (A): Final drive ratio (e.g., 3.08:1) - the driveshaft turns per single turn of the wheels.
• Drive gear teeth (N_drive): Number of teeth on the gear attached to the transmission output that turns the speedometer cable.
• Calibration constant (1,001): Standard reference that a typical mechanical speedometer is calibrated to 1,000 drive-shaft revolutions per indicated mile.

The constant 20,168 is derived. One mile equals 63,360 inches, and one tire revolution covers pi times the tire diameter in inches, so revolutions per mile = 63,360 / (pi x D). The 1,001 constant captures the convention that a mechanical speedometer is designed to read one mile per 1,000 drive-shaft revolutions.

The calculator first converts tire diameter to revolutions per mile, multiplies by axle ratio, multiplies by drive-gear teeth, and divides by 1,001. Real gears come in integer tooth counts, so the answer rounds to the nearest whole tooth.

According to Omni Calculator: Speedometer Gear, the standard formula is driven-gear teeth = axle ratio x tire revolutions per mile x drive-gear teeth / 1,001, and the 20,168 constant comes from 63,360 inches in a mile divided by pi.

To confirm the axle ratio number you typed into this calculator, the Axle Ratio Calculator works through the same drivetrain inputs in the opposite direction.

These four concepts are the building blocks of every mechanical speedometer. Knowing them helps you debug a wrong reading.

Putting the four concepts together explains why speedometer accuracy is so sensitive to tire changes. A 1-inch change in tire diameter changes revolutions per mile by 3 to 4 percent, which a 60 mph driver would feel as a 2 to 3 mph error.

The 1,001 constant is also why a single driven gear rarely fits more than one combination. If the tire diameter or axle ratio moves enough to push the calculated tooth count by even one tooth, the driven gear should be swapped.

When the tire sidewall gives a metric size like 265/70R17, the Tire Size Calculator converts it to the outside diameter in inches that this calculator expects.

Using the speedometer gear calculator is a four-step process. You supply three drivetrain numbers and the calculator returns the missing tooth count.

1. 1 Measure the tire diameter: Use the sidewall size to estimate the outside diameter in inches, or measure the mounted tire from the ground to the top of the tread and double it. Common passenger tires are 24 to 32 inches.
2. 2 Find the axle ratio: Check the door-jamb sticker, the differential tag, or the owner's manual. Common values are 2.73, 3.08, 3.23, 3.55, 3.73, and 4.10.
3. 3 Count the drive-gear teeth: Remove the cable or unscrew the driven-gear housing on the transmission to read the existing drive-gear tooth count. If missing, the dealer parts catalog lists it.
4. 4 Order the driven gear: The calculator returns the required driven-gear teeth rounded to the nearest whole tooth, the underlying revolutions per mile, and the final drive-to-driven ratio.

Suppose you installed 32-inch tires on a Jeep with a 3.73 axle ratio and a 9-tooth drive gear. Enter 32 for tire diameter, 3.73 for axle ratio, and 9 for drive-gear teeth. The calculator returns 630 revolutions per mile and a required driven-gear count of 21 teeth.

Once the driven gear is in place and the speedometer is reading correctly, the Gear Ratio Speed Calculator predicts the road speed at a given engine RPM with the new gearing.

Working through the math by hand is fast, but a calculator saves time, prevents mistakes, and lets you explore the effect of each variable.

• • Avoids trial-and-error gear swaps: Returns the exact driven-gear tooth count up front, so you buy the right part once instead of swapping gears to chase the reading.
• • Makes bigger-tire upgrades predictable: Lets you compare 32-, 33-, and 35-inch tires on the same vehicle and axle ratio to see the driven-gear change in advance.
• • Confirms the axle ratio: If the truck's history is unknown, the calculator can work backwards from a known driven-gear count and tire size to confirm the actual axle ratio.
• • Supports older vehicles: Mechanical speedometers cannot be reprogrammed the way modern electronic ones can, so a physical gear swap is the only fix.
• • Useful as a teaching tool: Shows the relationship between tire size, axle ratio, and gauge reading in a single screen.

On a working vehicle, the output is a sanity check: if the calculated driven teeth do not match the gear physically installed, you have a documented reason for the speedometer error.

On a project vehicle, the output is a planning tool. A target tire-and-axle combo can be checked against standard driven-gear tooth counts (usually 14 to 22) before money is spent.

The math is exact, but the answer is only as good as the numbers you put in. These factors change the real-world reading even when the calculator output is correct.

• • The calculator covers only cable-driven and gear-driven mechanical speedometers. Modern electronic speedometers that read a wheel-speed sensor are tuned in the ECU, not with a physical driven gear.
• • The 1,001 calibration constant is the long-standing standard for North-American mechanical speedometers. Some European and Japanese applications use 1,000 exactly; if your vehicle's manual specifies a different number, override the constant.

The driven-gear answer is rounded to the nearest whole tooth, but a real speedometer will still drift a percent or two even with the right gear - that drift is normal and is a function of tire growth at speed, tread wear, and cable slack.

If the calculated driven-gear tooth count comes out above about 22, standard parts may not cover that ratio. The typical fix is a step-up or step-down gear adapter that splits the ratio across two pairs of gears.

According to Wikipedia: Speedometer, a mechanical speedometer reads road speed from the rotation of a cable driven by the transmission output, with the calibration chosen to match the factory tire size and axle ratio; if either changes, the gauge needs a different driven gear to stay accurate.

If you are also changing the ring-and-pinion at the same time as the tires, the Differential Gear Ratio Calculator lets you match the new axle ratio to the available standard driven-gear tooth counts.