Gear Ratio and Speed Calculator

Gear Ratio and Speed Calculator

Overall Gear Ratio:

Vehicle Speed:

Understanding Gear Ratio and Vehicle Speed

The relationship between your engine's revolutions per minute (RPM), your vehicle's gearing, and the size of your tires directly determines your road speed. This calculator helps you understand how these critical factors interact to produce a specific vehicle speed.

What is Gear Ratio?

A gear ratio is a direct measure of the mechanical advantage provided by a gear train. In a vehicle, it's the ratio of the number of turns the driving gear makes to the number of turns the driven gear makes. A higher gear ratio (e.g., 4.10:1) means the engine turns more times for each rotation of the wheels, providing more torque but less top speed. A lower gear ratio (e.g., 3.08:1) means the engine turns fewer times for each wheel rotation, offering less torque but higher potential top speed.

In a typical vehicle, there are two primary gear ratios that combine to form the Overall Gear Ratio:

1. Transmission Gear Ratio: This is the ratio for the specific gear your transmission is currently in (e.g., 1st, 2nd, 3rd, overdrive). Each gear has a different ratio, allowing the engine to operate efficiently across various speeds and loads.
2. Final Drive Ratio (Differential Ratio): This is a fixed ratio found in the differential, which is the last set of gears before power is sent to the drive wheels. It's constant regardless of which transmission gear you're in.

The Overall Gear Ratio is calculated by multiplying the Transmission Gear Ratio by the Final Drive Ratio.

How Tire Diameter Affects Speed

The diameter of your tires plays a significant role in determining your vehicle's speed. A larger tire diameter means that for every single rotation of the wheel, the vehicle travels a greater distance. Conversely, smaller tires will cause the vehicle to travel a shorter distance per wheel rotation. This means that with larger tires, your vehicle will achieve a higher speed at the same engine RPM and gear ratio compared to smaller tires.

The Calculation Explained

Our calculator uses the following formula to determine vehicle speed in Miles Per Hour (MPH):

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

• Engine RPM: The rotational speed of your engine's crankshaft, measured in revolutions per minute.
• Tire Diameter (inches): The total height of your tire from the ground to the top, measured in inches.
• Overall Gear Ratio: The product of your Transmission Gear Ratio and Final Drive Ratio.
• 336: This is a constant used for unit conversion, specifically when calculating MPH with tire diameter in inches. It accounts for the conversion from minutes to hours, inches to miles, and the circumference of the tire (π).

Practical Applications

Understanding these relationships is crucial for:

• Performance Tuning: Selecting optimal gear ratios for racing, towing, or fuel efficiency.
• Tire Upgrades: Predicting how changing tire size will affect speedometer accuracy and vehicle performance.
• Engine Swaps: Matching new engines to existing drivetrains or planning new drivetrain components.
• Fuel Economy: Lower RPM at cruising speeds generally leads to better fuel efficiency.

Example Usage:

Let's say you have a vehicle with:

• Engine RPM: 3000
• Transmission Gear Ratio (in 4th gear, which is often 1:1): 1.00
• Final Drive Ratio: 3.73
• Tire Diameter: 26 inches

Using the calculator:

1. Overall Gear Ratio: 1.00 * 3.73 = 3.73:1
2. Vehicle Speed: (3000 * 26) / (3.73 * 336) = 78000 / 1253.28 ≈ 62.23 MPH

This calculator provides a quick and accurate way to determine your vehicle's speed based on these critical drivetrain components, helping you make informed decisions about modifications or simply understand your vehicle better.