TH350 Speedometer Gear Calculator

What is a TH350 Speedometer Gear Calculator?

A TH350 speedometer gear calculator is an essential tool for anyone maintaining or modifying a vehicle equipped with a General Motors Turbo-Hydramatic 350 (TH350) automatic transmission. This calculator helps you determine the correct driven speedometer gear to ensure your speedometer accurately displays your vehicle's speed. Accuracy is crucial for legal compliance, fuel economy tracking, and overall driving safety.

The TH350 transmission, popular in many classic GM cars and trucks from the 1960s through the 1980s, relies on a mechanical speedometer system. This system uses two gears: a drive gear (fixed on the transmission output shaft) and a driven gear (interchangeable, located in the tailshaft housing). When you change components like your tire size or axle ratio, the relationship between these gears needs to be adjusted to maintain speedometer accuracy. This calculator simplifies that adjustment process.

Who should use it? Anyone who has:

• Changed their tire size (e.g., larger off-road tires, smaller custom wheels).
• Swapped their rear axle or changed the axle gear ratio.
• Performed a TH350 transmission rebuild or swapped a transmission.
• Is experiencing an inaccurate speedometer reading and suspects an incorrect gear.

Common misunderstandings: Many assume that changing tires slightly won't affect the speedometer significantly, but even a small percentage difference in tire diameter can lead to several MPH error at highway speeds. Another common mistake is overlooking the drive gear; while often fixed, some TH350s had different drive gears, which also impact the calculation.

TH350 Speedometer Gear Formula and Explanation

The calculation for the correct TH350 speedometer driven gear is based on a simple ratio, ensuring that the speedometer cable rotates at the standard 1000 revolutions per mile (RPM) that GM speedometers are calibrated for. The formula takes into account your tire size, axle ratio, and the number of teeth on your transmission's drive gear.

Driven Gear Teeth = (Tire Revolutions Per Mile × Drive Gear Teeth × Axle Ratio) / Speedometer Cable Revolutions Per Mile

Where:

• Tire Revolutions Per Mile: This is the number of times your tire rotates to cover one mile. It's inversely proportional to your tire's diameter. Larger tires have fewer revolutions per mile.
• Drive Gear Teeth: The number of teeth on the plastic gear located on the transmission's output shaft. For TH350s, common counts are 15, 17, 18, 19, 20, or 21 teeth.
• Axle Ratio: The ratio of the number of turns of the driveshaft to the number of turns of the wheel axle. A higher number (e.g., 4.10) means more driveshaft rotations per wheel rotation, affecting speedometer input.
• Speedometer Cable Revolutions Per Mile: For most GM mechanical speedometers, this value is standardized at 1000.

Variables Table

The calculator first determines your tire's revolutions per mile using the formula: Tire Revs/Mile = 20168 / Tire Diameter (in inches). This constant 20168 is derived from the conversion of miles to inches and the circumference of a circle.

Practical Examples for TH350 Speedometer Gear Calculation

Let's walk through a couple of examples to illustrate how to use the TH350 speedometer gear calculator and interpret its results.

Example 1: Stock Setup with Minor Tire Change

Imagine you have a classic car with a TH350 transmission. Originally, it had 26-inch diameter tires and a 3.08 axle ratio. You've installed slightly larger 27.5-inch tires but kept the same 17-tooth drive gear in the transmission.

• Inputs:
  • Tire Diameter: 27.5 inches
  • Axle Ratio: 3.08
  • Drive Gear Teeth: 17
• Calculation Steps (Internal):
  1. Tire Revolutions Per Mile = 20168 / 27.5 ≈ 733.38 RPM
  2. Ideal Driven Gear Teeth = (733.38 × 17 × 3.08) / 1000 ≈ 38.30
• Result: The calculator would suggest a 38-tooth driven gear. If your old gear was, for example, 36 teeth, you'd need to upgrade to a 38-tooth gear for accuracy with the new tires.

Example 2: Performance Upgrade with Axle Swap

Suppose you're upgrading your muscle car with a TH350. You've swapped out the old 3.08 rear axle for a more aggressive 4.10 ratio and installed new 28-inch diameter tires. Your TH350 still has its original 19-tooth drive gear.

• Inputs:
  • Tire Diameter: 28 inches
  • Axle Ratio: 4.10
  • Drive Gear Teeth: 19
• Calculation Steps (Internal):
  1. Tire Revolutions Per Mile = 20168 / 28 ≈ 720.29 RPM
  2. Ideal Driven Gear Teeth = (720.29 × 19 × 4.10) / 1000 ≈ 56.09
• Result: The calculator would recommend a 56-tooth driven gear. This is a significantly larger gear, reflecting the combined impact of larger tires and a much higher axle gear ratio. Finding a 56-tooth gear might be challenging, indicating you might need to adjust your drive gear or consider an electronic speedometer adapter if an appropriate gear is unavailable.

How to Use This TH350 Speedometer Gear Calculator

Using this calculator is straightforward, designed to get you accurate results quickly.

1. Gather Your Data:
   • Tire Diameter: Measure the overall height of your installed tire from the ground to the top (or look up specifications for your tire size, e.g., using a tire size conversion tool).
   • Axle Ratio: Determine your vehicle's rear axle gear ratio. This can often be found on a tag on the differential cover, in your vehicle's service manual, or by counting gear teeth if you're rebuilding the differential.
   • TH350 Drive Gear Teeth: The drive gear is typically pressed onto the transmission output shaft. Common TH350 drive gear tooth counts are 15, 17, 18, 19, 20, or 21. If you're unsure, you might need to partially disassemble the tailshaft to inspect it, or assume a common 17-tooth gear for initial estimates.
2. Input the Values: Enter your collected data into the respective fields in the calculator above.
3. Select Correct Units: For "Tire Diameter," ensure you select the correct unit (inches, cm, or mm) that matches your measurement. The calculator will automatically convert it internally.
4. Calculate: Click the "Calculate Gear" button.
5. Interpret Results: The calculator will display the recommended driven gear teeth (rounded to the nearest whole number). It also shows intermediate values like "Tire Revolutions Per Mile" and the "Ideal Driven Gear Teeth" (exact decimal value) for your reference.
6. Copy Results: Use the "Copy Results" button to easily save or share your calculation details.
7. Reset: The "Reset" button will restore the default values, allowing you to perform new calculations quickly.

Key Factors That Affect TH350 Speedometer Accuracy

Achieving accurate speedometer readings with your TH350 transmission depends on several interconnected factors. Understanding these helps in troubleshooting and ensuring optimal performance.

1. Tire Diameter: This is arguably the most significant factor. Even a slight change in tire height dramatically alters the number of revolutions per mile, directly impacting the speedometer's input. Larger tires slow down the speedometer, while smaller tires speed it up.
2. Axle Ratio: Your rear axle's gear ratio determines how many times the driveshaft (and thus the transmission output shaft) spins for each wheel rotation. A "taller" (lower numeric) ratio like 2.73 will require a different driven gear than a "lighter" (higher numeric) ratio like 4.10, even with the same tires.
3. TH350 Drive Gear Teeth: While often overlooked, the drive gear on the transmission output shaft is a critical component. TH350s came with various drive gears (typically 15-21 teeth). If this gear is changed or incorrect, it will throw off all calculations.
4. Speedometer Cable and Head Unit: While the gear calculation focuses on the mechanical input, a worn or binding speedometer cable, or a faulty speedometer head unit, can also cause inaccurate readings regardless of correct gearing. Most GM speedometers are calibrated for 1000 cable RPM.
5. Transmission Tailshaft Housing: The tailshaft housing dictates the physical space and gear mesh for the driven gear. Some housings accommodate a wider range of driven gears than others. Ensure your housing can accept the calculated gear size.
6. Gear Availability: Sometimes, the perfectly calculated gear might not be available from manufacturers. In such cases, you might need to choose the closest available gear, accept a slight inaccuracy, or explore electronic speedometer correction modules.
7. Precision of Measurements: The accuracy of your inputs (especially tire diameter and axle ratio) directly affects the accuracy of the calculated driven gear. Always strive for precise measurements.

Frequently Asked Questions (FAQ) About TH350 Speedometer Gears

Q: What is the most common TH350 drive gear?

A: While there were several options, a 17-tooth drive gear is very common in many TH350 applications. However, it's always best to physically inspect your transmission or consult its specifications if possible.

Q: Can I use a TH350 speedometer gear calculator for other transmissions?

A: No, this calculator is specifically for the TH350 transmission because it assumes a standard 1000 speedometer cable revolutions per mile and specific drive gear options. Other transmissions (like the TH400, 700R4, 4L60E, or Ford transmissions) have different internal ratios and speedometer calibration standards, requiring their own specific calculators or methods for speedometer accuracy.

Q: What if the calculated driven gear isn't available?

A: If the exact tooth count isn't available, choose the closest available gear. For example, if 38.30 teeth are calculated, a 38-tooth gear is usually the best choice. For extremely precise needs, or if the closest gear still results in too much error, consider an electronic speedometer correction module.

Q: How does tire diameter affect my speedometer?

A: Larger tires cover more distance per revolution, making your speedometer read lower than your actual speed. Smaller tires cover less distance, making your speedometer read higher. This calculator helps compensate for these changes.

Q: Is my TH350's drive gear changeable?

A: The drive gear is pressed onto the output shaft and is generally considered a fixed component unless you're rebuilding the transmission. Changing it usually requires disassembling the transmission tailshaft and pressing off the old gear. Most adjustments are made by changing the more accessible driven gear.

Q: Why is my speedometer still inaccurate after changing the gear?

A: Several factors could be at play: incorrect initial measurements (tire diameter, axle ratio), a faulty speedometer cable, a worn or incorrect drive gear, or an issue with the speedometer head unit itself. Ensure all inputs were accurate and consider testing the GM automatic transmissions speedometer cable and head unit.

Q: What is the maximum number of teeth for a TH350 driven gear?

A: The maximum number of teeth for a TH350 driven gear depends on the tailshaft housing design. Typically, the range is from around 30 teeth up to about 45 teeth. Some specific housings (e.g., long tailshafts) might accommodate slightly more, but exceeding 45 teeth often requires custom solutions or electronic correction.

Q: Does the color of the speedometer gear matter?

A: Yes, for OEM GM gears, the color often corresponds to the tooth count. For example, a blue gear might be 38 teeth, a natural/white gear 40 teeth, etc. However, aftermarket gears may not follow this color coding, so always rely on the actual tooth count.

Related Tools and Internal Resources

Beyond finding the perfect TH350 speedometer gear, optimizing your vehicle's performance and accuracy often involves other calculations and guides. Explore these related resources:

• TH350 Transmission Rebuild Guide: A comprehensive guide to overhauling your Turbo-Hydramatic 350.
• Tire Size Calculator: Convert tire dimensions and compare different tire sizes easily.
• Axle Ratio Calculator: Understand how different axle ratios affect RPM and speed.
• Speedometer Calibration Guide: Learn more about calibrating your speedometer for various setups.
• GM Automatic Transmissions Explained: Dive deeper into the history and mechanics of GM's automatic gearboxes.
• Classic Car Performance Upgrades: Discover ways to enhance your classic vehicle's capabilities.