Semi-Truck Gear Ratio & Speed Calculator

Understanding your semi-truck's speed capability is essential for safe driving, especially during long journeys on highways. Our easy-to-use Speed Calculator assists truck owners and operators in estimating their speed based on Ring & Pinion Gear Ratio, RPM, Tire Diameter (mm), and Overdrive Gear Ratio. By inputting these parameters, our calculator swiftly computes the truck's speed in MPH.

Please note these calculators are provided for informational purposes only and unrelated to Drivewyze and its software products.

Visit Drivewyze PreClear ( if you are interested in the largest network of weigh station bypass sites in North America and Safety+ ( for a new way to coach drivers and improve safety outcomes.

Calculate Your Truck's Speed

Formula Used

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

In this formula:

  • RPM is the revolutions per minute of the engine.
  • Tire Diameter is the diameter of the tire in mm.
  • Overdrive Gear Ratio and Gear Ratio affect how the engine's power is translated to movement.


Suppose a truck has the following specifications:

  • Ring & Pinion Gear Ratio: 3.55
  • RPM: 2500
  • Tire Diameter: 1000mm (39.37 inches)
  • Overdrive Gear Ratio: 0.75

Using the formula, the calculated speed would be approximately 66.15 MPH.

Frequently Asked Questions (FAQs)

Why is knowing my truck's speed important?

Knowing the potential speed of your truck helps in planning long journeys, ensuring you drive within safe limits, and meeting trip schedule requirements.

How do the Gear Ratios affect my truck's speed?

Gear ratios play a significant role in determining how the engine's power is transferred to the wheels. A lower gear ratio can provide more power but less speed, while a higher gear ratio can offer greater speed but less power.

Is tire diameter crucial for calculating speed?

Yes, the tire diameter directly influences the distance a truck can travel in one tire revolution. Larger tires will cover more ground per revolution, potentially increasing the speed if all other variables remain constant.

How accurate is this calculator?

This calculator provides an estimate based on the input values provided. Real-world factors such as truck load, terrain, and engine efficiency can influence the actual speed.