Speedometer Gear Calculator

How to use this calculator

This calculator has two modes. Use the Find Required Gear tab when you have changed tire size and need to know what speedometer driven gear tooth count to buy. Use the Speed Correction tab when you want to calculate actual vehicle speed from known transmission output RPM, tire size, and axle ratio.

Find Required Gear tab:

• Original Tire Diameter: The overall diameter of the factory tire in decimal inches. If you have a tire size like 245/65R17, the diameter is (17 × 25.4 + 2 × 245 × 0.65) / 25.4 = 29.54 inches. Use the Tire Size Calculator to convert metric notation to inches if needed.
• New Tire Diameter: The overall diameter of the replacement tire in inches.
• Current Speedometer Gear Teeth: Count the teeth on your existing driven gear, or look up the factory specification in a service manual for your transmission, axle ratio, and original tire size.
• Speedometer Type: Select Mechanical for cable-driven speedometers. Select Electronic VSS for vehicles using a vehicle speed sensor with an instrument cluster that accepts a corrector device or has a physically swappable pulse generator gear.

Speed Correction tab:

• Tire Diameter: Overall tire diameter in inches.
• Axle Ratio: The rear (or front) differential gear ratio. Common ratios: 3.08, 3.31, 3.55, 3.73, 4.10. Find it on the axle tag or in the VIN decode.
• Transmission Output RPM: The rotational speed of the transmission output shaft. At highway cruise, this is typically engine RPM divided by the transmission gear ratio.

Mechanical speedometers: a brief history

The mechanical speedometer was standard equipment on virtually every passenger vehicle from the early 1900s through the early 1990s. Understanding how it works explains why the speedometer driven gear is the key calibration point.

The system starts with the transmission output shaft. As the vehicle moves, the output shaft rotates at a speed directly proportional to road speed and inversely proportional to the axle gear ratio. A small plastic drive gear is pressed or clipped onto the output shaft. This drive gear meshes with the speedometer driven gear, which is housed in a fitting that screws into the transmission tail housing.

The driven gear connects to the inner core of the speedometer cable. As the driven gear turns, it rotates the cable. The cable runs from the transmission up through the firewall to the back of the speedometer gauge head on the dashboard.

Inside the speedometer head, the rotating cable drives a horseshoe magnet. As the magnet spins, it induces eddy currents in an aluminum drag cup surrounding it. Those eddy currents create a magnetic drag force proportional to the rotation speed, which deflects the needle through a calibration spring. Higher cable speed equals more deflection equals higher indicated speed.

The system was simple, reliable, and required no electronics. Its weakness was sensitivity to gear ratio changes: any alteration to the drive/driven gear ratio or to tire circumference shifted the calibration.

The drive gear and driven gear system

The ratio between the drive gear (on the output shaft) and the driven gear (in the tail housing) determines how many cable revolutions occur per output shaft revolution. Combined with tire circumference and axle ratio, this sets the calibration.

Most manufacturers chose a system where the speedometer cable makes approximately 1,000 revolutions per mile at correct calibration. The drive and driven gear tooth counts were selected to achieve that target for a specific axle ratio and tire size combination.

This formula works because the driven gear tooth count is directly proportional to how many driven gear rotations occur per tire revolution. If the new tire has a larger circumference, the output shaft turns fewer times per mile, so the driven gear also turns fewer times. To maintain the 1,000 rev/mile target, you need more teeth on the driven gear to compensate for the slower shaft rotation.

A larger tire needs a higher tooth count. A smaller tire needs a lower tooth count.

Electronic VSS systems

Beginning in the late 1980s and becoming universal by the mid-1990s, electronic vehicle speed sensors replaced the mechanical cable. The VSS generates electrical pulses as a toothed reluctor ring or tone wheel rotates past the sensor. The instrument cluster or ECU counts these pulses per second and converts them to a speed reading using a calibration constant.

There are two common VSS locations:

1. Transmission-mounted VSS: Located in the tail housing, driven by the same output shaft gear as the mechanical cable was. On many vehicles, a physical plastic gear still drives a pulse generator. Others use a magnetic reluctor ring pressed directly onto the output shaft.

2. Wheel speed sensors: On vehicles with ABS, the ABS sensors at each wheel hub double as VSS inputs. The instrument cluster averages the four wheel speeds (or uses a specific wheel’s signal) for the speedometer. This is the most common arrangement on vehicles built after 2000.

For transmission-mounted VSS with a physical gear, the same tooth-count calculation applies. For wheel speed sensor based systems, there is no physical gear to swap. Correction requires either ECU reprogramming or an inline electronic corrector device.

Calculating speed from transmission output RPM

The Speed Correction tab calculates vehicle speed directly from mechanical inputs. This is useful for diagnosing calibration issues, setting up a new drivetrain, or verifying that a gear ratio change (axle swap) gives the correct top speed.

Breaking this down:

• Tire Diameter × π: Tire circumference in inches (distance per revolution)
• Trans Output RPM × 60: Output shaft revolutions per hour
• Divide by Axle Ratio: Converts from transmission output revolutions to wheel revolutions
• Divide by 63,360: Converts from inches per hour to miles per hour (63,360 inches = 1 mile)

Where to buy speedometer gears and what to order

When ordering a replacement speedometer gear, you need three pieces of information: the transmission model, the driven gear housing color (which affects which gear fits the housing bore), and the required tooth count from the calculator.

Common transmission families and their gear ranges:

Where to buy:

• NAPA Auto Parts and O’Reilly carry common tooth counts for popular transmissions (usually 17-25 teeth stocked locally)
• Summit Racing and Jegs stock a wider range including high tooth counts for large tire applications
• Transmission specialty suppliers like Transmissionpartsusa.com carry the broadest selection
• GM dealers still supply gears for classic trucks at reasonable prices

Always verify tooth count by counting the teeth on the gear, not by relying solely on the part number or color code. Aftermarket gears may not follow the original manufacturer’s color convention.

Installing a speedometer gear: step by step overview

On most GM and Ford trucks with the 700R4, 4L60E, or similar transmission, the driven gear is accessible from the driver side of the transmission tail housing without dropping the transmission. The process typically takes 30-45 minutes.

1. Raise and support the vehicle safely on jackstands. Work on a level surface.

2. Locate the speedometer driven gear housing. It is a small fitting screwed into the tail housing on the driver side, with the speedometer cable or VSS connector attached to it.

3. Disconnect the cable or connector. For a cable speedometer, unscrew the cable fitting. For a VSS, unplug the electrical connector.

4. Remove the retaining clip and bolt. Most housings are held by a single bolt and a small retaining clip. Remove both and pull the housing straight out. A small amount of transmission fluid may drip out, so have a rag ready.

5. Remove the driven gear from the housing. The gear is typically held on the shaft by a clip inside the housing. Remove the clip, slide off the old gear, slide on the new gear, and reinstall the clip.

6. Reinstall the housing. Push it straight in, align the slot with the retaining clip groove, install the clip and bolt.

7. Reconnect the cable or VSS connector.

8. Test: Drive at a known speed (verify with GPS) and confirm the speedometer is now accurate.

Electronic correction alternatives

For vehicles where a physical gear swap is not possible or practical, electronic speed correctors provide a clean solution. These devices intercept the VSS signal and multiply it by a programmable factor before passing it to the instrument cluster.

Dakota Digital SGI-5E: One of the most popular units, priced around $80. It intercepts the VSS pulse signal, multiplies it by a factor you program (via a small button), and outputs the corrected signal to the cluster. Compatible with most OEM speed signal formats. Installation takes about 2 hours with basic wiring skills.

SpeedHut SpeedoCorrector: Similar function, slightly lower price point. Works well on GM applications.

Hypertech Speedometer Calibrator: Available for many GM and Ford trucks. Plugs into the OBD-II port and reprograms the speedometer calibration directly in the ECU. No wiring required. Particularly clean solution for 2000s-era GM trucks.

HP Tuners / EFI Live: Full-featured ECU tuning software that, among other things, lets you enter the exact tire circumference for perfect speedometer calibration. Requires purchasing credits or a license. Most appropriate if you are already having other tuning done.

The electronic corrector approach is ideal for: modern unibody vehicles without accessible speedometer gears, vehicles where the required tooth count falls outside the available range (over 45 or under 15), and applications where you want an adjustable correction for future tire size changes without buying new gears each time.

Common vehicles with swappable speedometer gears

The following vehicles are among the most common candidates for a speedometer gear swap after tire size upgrades. All use the transmission-mounted driven gear system.

GM trucks (C/K 1500/2500/3500, 1988-2007): The 700R4, 4L60E, and 4L80E transmissions all use swappable plastic driven gears. This is the most well-documented application. Available gears cover 17-45 teeth, covering virtually any practical tire size and axle ratio combination.

Ford F-150 (1990-2004): The AOD and 4R70W transmissions use a similar system. Ford gears are available in a more limited range (18-22 teeth) from dealers, with wider availability from specialty suppliers.

Jeep Cherokee and Wrangler (YJ/XJ/TJ): The AX-15 and NV3550 manual transmissions and 32RH automatic use swappable gears. Very commonly upgraded due to the popularity of larger tire fitments on these platforms.

Dodge Ram 1500/2500 (1994-2002): The A518/46RH and 46RE transmissions use a driven gear approach. Less widely documented than the GM system but parts are available.

Ford Bronco and F-150 (pre-1990): The C6 and AOD use driven gears. The older C6 has a limited range of available replacement gears.

Always cross-reference the exact transmission code (found on the transmission ID tag) with a gear supplier’s application guide before ordering. The same vehicle model may have come with different transmissions in different years.