LMR Speedometer Gear Calculator
Calibrate your Mustang’s speedometer by calculating the required driven gear teeth based on your axle ratio and tire dimensions.
The number of teeth on the gear inside your transmission tailshaft.
Common Ford ratios: 2.73, 3.08, 3.27, 3.55, 3.73, 4.10.
Please enter a valid positive ratio.
Format: Width / Aspect / Rim (e.g., 245/45R17)
Ensure all tire measurements are valid.
Formula: (Drive Teeth × Axle Ratio × Tire Revs) / 1000
25.68 inches
785.35
19 Tooth (Tan)
Driven Gear Requirements by Axle Ratio
This chart displays how the required tooth count changes as you increase your rear end gear ratio.
| Teeth Count | Color Code | Ford Part Number | Application Type |
|---|---|---|---|
| 16 | Wine | C0DZ-17285-C | Manual/Auto |
| 17 | White | C3DZ-17285-C | Manual/Auto |
| 18 | Yellow | C0DD-17285-B | Manual/Auto |
| 19 | Pink/Tan | C0DZ-17285-B | Manual/Auto |
| 20 | Black | C1DZ-17285-A | Manual/Auto |
| 21 | Red | C4OZ-17285-C | Manual/Auto |
| 23 | White (Aftermarket) | LMR-17285-23 | High Ratio Only |
What is an lmr speedometer gear calculator?
An lmr speedometer gear calculator is a specialized automotive calibration tool used by Ford Mustang and late-model Ford enthusiasts to correct speedometer inaccuracies. When a vehicle undergoes modifications such as a rear-end gear ratio swap (e.g., going from 2.73 to 3.73 gears) or a change in tire diameter, the speedometer signal becomes uncalibrated. The lmr speedometer gear calculator determines the precise number of teeth required on the cable-driven speedometer gear located in the transmission tailshaft to restore factory accuracy.
Who should use it? Anyone driving a Mustang from 1979 through 1998, or any Ford vehicle utilizing the T5, AOD, or 4R70W transmissions with a mechanical speedometer drive. A common misconception is that the lmr speedometer gear calculator is only for professional mechanics; however, it is a vital tool for any DIY hobbyist looking to avoid speeding tickets and keep their odometer readings honest.
lmr speedometer gear calculator Formula and Mathematical Explanation
The mathematics behind the lmr speedometer gear calculator relies on the relationship between mechanical rotations and distance. To find the correct driven gear, we use a standard automotive derivation that considers the drive gear, the final drive ratio, and the tire’s rolling circumference.
The Core Formula:
Driven Gear Teeth = (Drive Gear Teeth × Axle Ratio × Tire Revolutions Per Mile) / 1000
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Drive Gear | Teeth on the transmission internal shaft | Count | 6, 7, or 8 |
| Axle Ratio | Ring and pinion gear ratio | Ratio | 2.73 to 4.56 |
| Tire Revs | Number of tire rotations in one mile | Revs | 700 to 900 |
Practical Examples (Real-World Use Cases)
Example 1: The Classic 3.73 Gear Swap
A user with a 1993 Mustang GT swaps their stock 2.73 gears for 3.73 performance gears. They have a 7-tooth drive gear and standard 245/45R17 tires (approximately 811 revs per mile). Using the lmr speedometer gear calculator: (7 × 3.73 × 811) / 1000 = 21.17. The user should install a 21-tooth Red gear to achieve nearly perfect accuracy.
Example 2: Oversized Drag Radials
A racer installs tall 28-inch drag tires on a car with 4.10 gears and an 8-tooth drive gear. A 28-inch tire has roughly 720 revs per mile. The lmr speedometer gear calculator provides: (8 × 4.10 × 720) / 1000 = 23.61. In this case, a 23-tooth aftermarket white gear is the closest fit, though a slight error may persist due to the physical limits of gear sizing.
How to Use This lmr speedometer gear calculator
Following these steps ensures you get the most out of the lmr speedometer gear calculator for your Ford project:
- Identify Drive Gear: Check your transmission type to find the internal tooth count (usually 7 for late-model T5s).
- Enter Axle Ratio: Input the gear ratio installed in your differential housing.
- Tire Dimensions: Enter your sidewall numbers (Width/Aspect/Rim) to let the lmr speedometer gear calculator determine your tire height.
- Review Results: Look at the “Required Driven Gear Teeth” and round to the nearest whole number.
- Match Color: Use the reference table to find the corresponding gear color to purchase.
Key Factors That Affect lmr speedometer gear calculator Results
Several variables can influence the precision of your lmr speedometer gear calculator outputs:
- Tire Pressure: Low pressure reduces the effective rolling radius, increasing revolutions per mile and skewing the lmr speedometer gear calculator results.
- Tread Wear: A bald tire is smaller than a new one, which can cause a 1-2% variance in speedometer readings over time.
- Centrifugal Growth: At high speeds, tires can “grow” in height, a factor the lmr speedometer gear calculator assumes is constant.
- Drive Gear Variance: Some transmissions were built with different internal tooth counts depending on the year; verifying this is critical for the lmr speedometer gear calculator.
- Manufacturing Tolerances: Not all gears are cut exactly the same, which is why the lmr speedometer gear calculator provides the closest match possible.
- Transmission Swaps: If you’ve swapped a Tremec or a different AOD into your car, the internal drive gear may have changed, requiring a re-run of the lmr speedometer gear calculator.
Frequently Asked Questions (FAQ)
Related Tools and Internal Resources
- Mustang Axle Ratio Chart – Determine your stock rear end ratio.
- T5 Transmission Identification – Learn how to identify your drive gear count.
- Tire Size Comparison Tool – Compare height differences between tire sets.
- Ford Speedometer Gear Color Code – A visual guide to gear colors and part numbers.
- Electronic Speedometer Calibration – For 1999-present Mustang owners.
- Mustang Performance Parts – Shop for the gears identified by our lmr speedometer gear calculator.