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Speedometer Gear Calculator

Calculate the correct speedometer driven gear tooth count for your transmission after changing tire size, rear axle ratio, or drive gear teeth.

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Required Driven Gear Teeth

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Required Driven Gear Teeth—teeth

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What Is a Speedometer Gear Calculator?

A speedometer gear calculator determines the exact number of teeth required on the speedometer driven gear — the small plastic or nylon gear mounted on the transmission tailshaft that spins the speedometer cable. Any modification that changes how fast the driveshaft rotates relative to actual road speed — new rear axle gears, larger or smaller tires, or a transmission swap — can throw the factory speedometer reading off by 10–25% or more. This calculator solves that problem precisely using a formula derived from tire geometry and drivetrain constants.

The Speedometer Driven Gear Formula

The required tooth count on the driven gear is calculated as:

Ndriven = (Ndrive × Raxle × (63,360 ÷ (π × Dtire))) ÷ Cspeedo

The constant 63,360 represents the number of inches in one statute mile (5,280 feet × 12 inches per foot). Dividing it by the tire circumference (π × Dtire) converts the measurement into tire revolutions per mile. Multiplying by the axle ratio and drive gear tooth count yields the total number of drive gear rotations the transmission output shaft completes per mile. Dividing by the transmission speedometer cable constant produces the driven gear tooth count that makes the speedometer cable spin at exactly the correct rate — the value that aligns cable speed with indicated vehicle speed, as documented by TCI Automotive's speedometer gear selection guide and corroborated by Summit Racing's technical speedometer gear reference.

Understanding Each Variable

  • Tire Diameter (Dtire): The overall loaded diameter of the rear tire in inches. A 275/60R17 tire, for example, measures approximately 30.0 inches in overall diameter. Convert metric sidewall codes using: (section width in mm × aspect ratio × 2 ÷ 25.4) + rim diameter in inches.
  • Rear Axle Ratio (Raxle): The ring-and-pinion ratio of the rear differential. A ratio of 3.73:1 means the driveshaft makes 3.73 full rotations for each single rotation of the axle shafts. Common performance ratios range from 2.73 to 4.88 and are typically stamped on the differential cover or listed on the vehicle build sheet.
  • Drive Gear Teeth (Ndrive): The number of teeth on the drive gear pressed onto the transmission output shaft. Most General Motors automatic transmissions use a 7-tooth drive gear from the factory; some applications use 6 or 8 teeth. Always remove and count the installed gear teeth directly rather than assuming the factory count.
  • Speedometer Cable Constant (Cspeedo): The number of speedometer cable revolutions per mile that a given transmission family requires for a correct reading. The GM Turbo-Hydramatic 350 targets approximately 1,001 rev/mile, the TH400 uses 1,000, and the 700R4 (4L60) also uses 1,000, as cross-referenced in the Bowtie Overdrives 700R4 speedometer gear chart. Ford C4 and C6 transmissions similarly target 1,000 rev/mile.

Real-World Calculation Example

Consider a GM vehicle with a 700R4 transmission (Cspeedo = 1,000 rev/mile), a 7-tooth drive gear, a 3.73:1 rear axle ratio, and 30.0-inch-diameter tires:

  • Tire revolutions per mile: 63,360 ÷ (3.14159 × 30.0) ≈ 672.3 rev/mile
  • Drive gear rotations per mile: 672.3 × 3.73 × 7 ≈ 17,568
  • Required driven gear teeth: 17,568 ÷ 1,000 ≈ 17.6 → round to 18 teeth

Because driven gears are manufactured only in whole-tooth increments — typically 15 to 45 teeth depending on the application — always round the calculated result to the nearest available tooth count. Rounding to the nearest integer keeps speedometer error within 1–3%, well inside the ±4% tolerance most states permit for street-driven vehicles.

When to Recalculate the Speedometer Gear

Recalculation is necessary after any of the following modifications: installing tires with a different overall diameter than the factory tires, changing the rear differential ring-and-pinion gears to a numerically higher or lower ratio, swapping to a different transmission family, or replacing a worn drive gear with one having a different tooth count. Even switching from a standard-load to an extra-load tire of the same nominal size can shift the loaded diameter by 0.3–0.5 inches, moving the required driven gear count by one tooth and introducing a measurable speedometer error over time.

Reference

Frequently asked questions

What happens if I install the wrong speedometer driven gear?
An incorrect driven gear produces a speedometer reading that is consistently too high or too low at every speed. For example, a gear with two extra teeth causes the speedometer to read approximately 10% high, making the vehicle appear to travel at 66 mph when it is actually moving at 60 mph. Beyond simple inconvenience, inaccurate readings can result in speeding citations and odometer-based maintenance intervals that come due too early or too late, shortening engine life or voiding warranty coverage.
How do I measure my tire's overall diameter for the speedometer gear formula?
Convert the three-part metric code on the tire sidewall using the formula: overall diameter equals (section width in millimeters times aspect ratio times 2 divided by 25.4) plus rim diameter in inches. A 275/60R17 tire calculates to (275 times 0.60 times 2 divided by 25.4) plus 17, which equals 30.0 inches. An alternative field method involves marking a reference point on the tire, rolling the vehicle exactly one full tire revolution on a level surface, and measuring the distance traveled in inches, then dividing by pi (3.14159) to derive the diameter.
What drive gear tooth counts are most common on GM and Ford automatic transmissions?
Most General Motors automatic transmissions — including the TH350, TH400, 700R4, and 4L60E — use a 7-tooth drive gear as the factory-standard option, though 6-tooth and 8-tooth drive gears exist for certain applications. Ford C4 and C6 transmissions also most commonly use 7 teeth. Always remove and physically count the installed drive gear teeth rather than assuming the factory configuration, since a previous builder or owner may have installed a non-standard gear during a prior repair or performance modification.
What is the speedometer cable constant and where do I find the correct value?
The speedometer cable constant is the number of full cable revolutions per mile that a specific transmission requires to display an accurate speed reading. For most GM and Ford automatics the value is 1,000 revolutions per mile, though some TH350 variants use 1,001. Find the correct constant in the transmission manufacturer's service manual, technical data sheets from suppliers such as TCI Automotive or Bowtie Overdrives, or on the transmission builder's specification card included with a remanufactured unit. Using the wrong constant shifts every calculated tooth count by a fixed percentage.
Can swapping the speedometer driven gear fix a speedometer that reads too fast or too slow?
Yes. A speedometer that consistently reads too fast — for example, showing 70 mph when GPS confirms 65 mph — needs a driven gear with fewer teeth to slow the cable rotation. A speedometer that reads too slow needs more teeth to speed the cable up. The speedometer gear calculator identifies the mathematically correct tooth count based on actual tire diameter, axle ratio, and drive gear teeth, eliminating repeated trial-and-error gear swaps and reducing the labor time required to achieve an accurate reading.
What range of speedometer driven gear tooth counts is typically available?
For most GM automatic transmissions, replacement speedometer driven gears are available from approximately 15 to 45 teeth in one-tooth increments, covering a wide range of tire size and axle ratio combinations. Ford applications offer a similar but slightly narrower range. TCI Automotive, Summit Racing, and local transmission suppliers stock the most frequently needed counts — roughly 17 to 35 teeth — and can special-order outliers. On many GM applications, driven gears are color-coded by tooth count (for example, white for 21 teeth, green for 22), making post-installation verification straightforward.