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Gear Ratio Calculator

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Created by: Ethan Brooks

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This Gear Ratio Calculator helps you determine the gear ratio, output speed (RPM), and output torque (assuming 100% efficiency) from the number of teeth on the driving and driven gears. You can optionally provide input speed and torque for more detailed calculations.

What is a Gear Ratio Calculator?

A Gear Ratio Calculator helps automotive enthusiasts, mechanics, and racers determine vehicle speed at any RPM across all transmission gears. By entering your transmission gear ratios, axle ratio, and tire size, you can instantly see how fast your vehicle will travel in each gear at a given engine RPM.

This tool is essential for selecting the right transmission and rear axle combination for your performance goals, whether you're building a drag car, road racer, or daily driver. Understanding gear ratios helps optimize acceleration, top speed, and fuel economy.

The calculator includes presets for popular transmissions like Tremec TKO, T-56, Magnum, GM Muncie, Ford Toploader, and more. You can also enter custom ratios for any transmission or differential combination.

Vehicle Speed Formula

Speed from RPM

Speed (MPH) = (RPM × Tire Diameter) / (Gear Ratio × Axle Ratio × 336)

Where:

  • RPM = Engine revolutions per minute
  • Tire Diameter = Overall tire diameter in inches
  • Gear Ratio = Transmission gear ratio for selected gear
  • Axle Ratio = Final drive (differential) ratio
  • 336 = Constant for conversion (derived from π and unit conversions)

RPM from Speed (Reverse Calculation)

RPM = (Speed × Gear Ratio × Axle Ratio × 336) / Tire Diameter

Use this formula to determine what RPM your engine will be turning at a specific speed in any gear.

Final Drive Ratio

Final Drive = Gear Ratio × Axle Ratio

The final drive ratio is the total mechanical advantage from engine to wheels in a given gear.

Example Calculation

A Mustang with a Tremec T-56, 3.73 rear axle, and 26" tires at 6000 RPM in 3rd gear:

  • 3rd Gear Ratio = 1.30
  • Axle Ratio = 3.73
  • Tire Diameter = 26 inches
  • Engine RPM = 6000

Speed = (6000 × 26) / (1.30 × 3.73 × 336) = 95.7 MPH

Common Applications

  • Transmission Selection: Compare how different transmissions affect vehicle speed and RPM characteristics.
  • Axle Ratio Optimization: Find the ideal rear gear ratio for your driving style and power band.
  • Drag Racing: Calculate trap speeds and determine if you'll hit the limiter before the finish line.
  • Road Racing: Ensure proper gearing for corner exit speeds and straightaway performance.
  • Cruising RPM: Check highway RPM to optimize fuel economy and engine longevity.
  • Tire Size Changes: See how different tire sizes affect your speedometer accuracy and gearing.

Frequently Asked Questions

What gear ratio is best for drag racing?

For drag racing, you want a ratio that lets you use your entire power band without hitting the rev limiter before the finish line. Typically 4.10-4.56 axle ratios work well with most 5/6-speed transmissions for quarter-mile racing.

How does tire size affect my gearing?

Larger tires effectively lower your gearing (like a numerically lower axle ratio), reducing RPM at a given speed. Smaller tires raise your effective gearing, increasing RPM at speed. A 1" change in tire diameter is roughly equivalent to a 0.10 change in axle ratio.

What RPM should I cruise at on the highway?

Most engines are most efficient cruising at 2000-2500 RPM. Check your gear ratio combination to ensure your top gear provides comfortable highway RPM. Too low causes lugging; too high wastes fuel and increases wear.

How do I choose between a close-ratio and wide-ratio transmission?

Close-ratio transmissions keep the engine in the power band during spirited driving but may have poor highway manners. Wide-ratio boxes offer better street driveability with greater gaps between gears. Match your transmission to your primary use.

What's the difference between overdrive and direct drive?

Direct drive is typically 4th gear in most transmissions (1.00:1 ratio). Overdrive gears have ratios below 1.00 (like 0.63:1), making the driveshaft turn faster than the engine for lower highway RPM.

Will changing my axle ratio affect my speedometer?

Yes, changing axle ratio will make your speedometer read incorrectly. Electronic speedometers can be recalibrated. Mechanical speedometers require a different speedometer gear or a correction box.

Tips for Gear Selection

  • Match gearing to power band: Your axle ratio should let you use peak power in the gears you use most
  • Consider your use case: Street cars need good highway manners; race cars prioritize acceleration
  • Check 1st gear ratio: Very steep 1st gears can cause wheelspin or be difficult to launch
  • Verify top speed: Ensure your gearing allows your target top speed before the rev limiter
  • Calculate shift points: Optimal shift point is where the next gear intersects your power curve
  • Plan for tire changes: If you change tire size seasonally, check how it affects your gearing

Sources and References

  1. TREMEC Corporation. "Gear Ratio Calculator and Transmission Specifications." tremec.com, 2024.
  2. Society of Automotive Engineers. "Manual Transmission Design and Selection." SAE International, 2023.
  3. Gillespie, T. D. "Fundamentals of Vehicle Dynamics." SAE International, 2021.