Lmr Speedometer Gear Calculator

Calculate Your Speedometer Gear

Changing your Mustang’s rear gears or tire size? Use this lmr speedometer gear calculator to find the correct driven gear and ensure your speedometer reads accurately.

---

Calculation Details

Metric	Value
Calculated Tire Diameter	—
Tire Revolutions Per Mile	—
Calculated Driven Gear (Exact)	—
Current Speedometer Error	—

What is an lmr speedometer gear calculator?

An lmr speedometer gear calculator is a specialized tool designed for vehicle owners, particularly of Ford Mustangs, to determine the correct speedometer “driven gear” required for accurate speed readings. When a car’s factory configuration is altered, such as by installing a different rear axle gear ratio or changing the overall diameter of the tires, the mechanical speedometer will no longer be accurate. This is because the speedometer’s reading is calibrated based on a specific number of tire revolutions per mile. The lmr speedometer gear calculator takes your new setup’s variables—tire dimensions, rear gear ratio, and transmission drive gear—and computes the ideal number of teeth for the plastic driven gear that connects to the speedometer cable. Installing the correct gear ensures your speedometer and odometer reflect your true speed and distance traveled. Anyone with a mechanically driven speedometer (common in cars like the Fox Body and SN95 Mustang, pre-1999) who has modified their drivetrain or wheels should use this tool. A common misconception is that any gear will work; however, being off by even one or two teeth can lead to significant speed reading errors and potential speeding tickets.

lmr speedometer gear calculator Formula and Mathematical Explanation

The core of the lmr speedometer gear calculator is a formula that aligns the transmission’s output with the standard speedometer calibration of 1000 cable revolutions per mile. The calculation determines how many teeth the driven gear needs to correctly reduce the rotation from the drive shaft to what the speedometer expects. The formula is as follows:

Driven Gear Teeth = (Drive Gear Teeth × Rear Axle Ratio × Tire Revolutions Per Mile) / 1000

The process starts by calculating the tire’s revolutions per mile, which is derived from its diameter. The step-by-step logic is:

1. Calculate Sidewall Height: (Tire Width × Aspect Ratio / 100) / 25.4
2. Calculate Overall Tire Diameter: (Sidewall Height × 2) + Wheel Diameter
3. Calculate Tire Revolutions per Mile: 20168 / Tire Diameter
4. Apply the Main Formula: Plug all the values into the primary formula shown above to get the required number of teeth for the driven gear.

Variables in the Speedometer Gear Calculation

Variable	Meaning	Unit	Typical Range
Drive Gear Teeth	The number of teeth on the gear fixed to the transmission output shaft.	Teeth	6, 7, or 8
Rear Axle Ratio	The ratio of the driveshaft’s revolutions to the axle’s revolutions.	Ratio	2.73 – 4.56
Tire Revolutions Per Mile	How many full rotations the tire makes in one mile.	Revolutions	700 – 900
Driven Gear Teeth	The resulting number of teeth needed for the removable speedometer gear.	Teeth	16 – 23

Practical Examples (Real-World Use Cases)

Example 1: Upgrading Rear Gears

A user owns a 1989 Mustang with a 7-tooth drive gear, stock 2.73 rear gears, and 225/60R15 tires. They upgrade to a performance-oriented 3.73 rear axle ratio. Their speedometer is now reading much higher than their actual speed. Using the lmr speedometer gear calculator, they input the new 3.73 ratio. The calculator determines that they need to switch from their original 18-tooth gear to a 21-tooth driven gear to correct the speedometer.

Example 2: Installing Larger Wheels and Tires

Another user has a 1995 Mustang with an 8-tooth drive gear and 3.27 rear gears. They replace the stock 225/55R16 tires (25.7″ diameter) with larger 275/40R17 tires (25.7″ diameter). In this specific case, because the overall tire diameter remains the same, the lmr speedometer gear calculator would show that their existing speedometer gear is still correct and no change is needed. However, if they had chosen a taller tire, such as a 275/40R18 (26.7″ diameter), the calculator would indicate they need a gear with fewer teeth (e.g., changing from a 21-tooth to a 20-tooth gear) to compensate for the taller tire’s slower rotation.

How to Use This lmr speedometer gear calculator

1. Enter Drive Gear Teeth: Select your transmission’s drive gear tooth count from the dropdown. If unsure, 7-tooth is common for Fox Body Mustangs and 8-tooth for early SN95s.
2. Enter Rear Axle Ratio: Type in the gear ratio of your car’s differential.
3. Enter Tire Dimensions: Input your tire’s width, aspect ratio, and the diameter of your wheel. This information is found on the tire’s sidewall.
4. Review the Result: The calculator will instantly display the recommended number of teeth for your new driven speedometer gear. Since gears are only available in whole tooth counts, you should choose the closest available gear. For example, if the result is 20.8, a 21-tooth gear is the best choice.
5. Check Intermediate Values: The results table provides extra details like your tire’s revolutions per mile and the exact calculated tooth count, helping you understand the math behind the recommendation.

Key Factors That Affect lmr speedometer gear calculator Results

• Drive Gear Teeth: This is the foundation of the calculation. A different drive gear (e.g., 7-tooth vs. 8-tooth) will require a completely different driven gear for the same setup.
• Rear Axle Ratio: This is a major multiplier in the formula. A higher numerical ratio (like 4.10) will spin the driveshaft faster, requiring a driven gear with more teeth to compensate.
• Tire Diameter: This is the most complex factor, as it’s a combination of width, aspect ratio, and wheel size. A taller tire covers more ground per revolution, so it requires a driven gear with fewer teeth. Even small changes in diameter can affect accuracy.
• Transmission Type: While not a direct input in this simplified calculator, some transmissions (like the T45 or AODE) are associated with specific drive gear counts from the factory.
• Tire Wear: As a tire wears down, its diameter decreases slightly. This can cause the speedometer to read slightly higher than the actual speed over time.
• Rounding: Since you can’t have a fraction of a tooth, you always have to round to the nearest whole number available for purchase. This can introduce a very small, generally negligible, margin of error.

Frequently Asked Questions (FAQ)

What if the lmr speedometer gear calculator gives a fractional result like 19.7?

You should round to the nearest whole number. In this case, a 20-tooth gear would be the ideal choice. Generally, rounding up will make your speedometer read slightly lower, and rounding down will make it read slightly higher.

Where is the speedometer driven gear located?

It’s located on the tailshaft of the transmission, where the speedometer cable or Vehicle Speed Sensor (VSS) plugs in. It is held in by a single bolt and a retaining clip.

Will this calculator work for newer cars with electronic speedometers (1999+ Mustangs)?

No. This lmr speedometer gear calculator is for vehicles with a mechanical, gear-driven speedometer. For 1999 and newer Ford Mustangs, speedometer correction must be done electronically with a handheld tuner.

How do I know how many teeth my transmission’s drive gear has?

The most accurate way is to remove the driven gear assembly and look inside the transmission tailshaft housing to count the teeth. The gear is also often color-coded. For many classic Ford transmissions, a 7-tooth gear is yellow or natural, and an 8-tooth is green.

Can changing tire pressure affect the speedometer?

Yes, but minimally. Lower pressure will slightly decrease the tire’s effective rolling diameter, causing the speedometer to read a tiny bit high. The effect is usually too small to notice without precise measurement.

Is it difficult to change a speedometer driven gear?

No, it’s typically a very easy job that takes only a few minutes. It involves removing one bolt, pulling out the cable and gear assembly, swapping a small clip to the new gear, and reinstalling.

Why does my speedometer read faster than my GPS?

Most manufacturers calibrate speedometers to read slightly high from the factory to prevent liability. Using this lmr speedometer gear calculator after a modification helps bring that reading much closer to the true speed.

What happens if I use the wrong gear?

Your speedometer and odometer will be inaccurate. If you use a gear with too few teeth, your speedometer will read lower than your actual speed, risking a speeding ticket. If you use a gear with too many teeth, it will read higher than your actual speed, and your odometer will accumulate miles too quickly.