Gear Ratio Calculator Rpm Speed

Gear Ratio, RPM, and Speed Calculator

function calculateSpeed() { var engineRPM = parseFloat(document.getElementById('engineRPM').value); var transmissionRatio = parseFloat(document.getElementById('transmissionRatio').value); var finalDriveRatio = parseFloat(document.getElementById('finalDriveRatio').value); var tireDiameter = parseFloat(document.getElementById('tireDiameter').value); var resultDiv = document.getElementById('result'); if (isNaN(engineRPM) || isNaN(transmissionRatio) || isNaN(finalDriveRatio) || isNaN(tireDiameter) || engineRPM <= 0 || transmissionRatio <= 0 || finalDriveRatio <= 0 || tireDiameter <= 0) { resultDiv.innerHTML = "Please enter valid positive numbers for all fields."; return; } var overallGearRatio = transmissionRatio * finalDriveRatio; // Formula: Speed (MPH) = (RPM * Tire Diameter * Pi) / (Overall Gear Ratio * 12 inches/foot * 5280 feet/mile * 60 minutes/hour) // Simplified: Speed (MPH) = (RPM * Tire Diameter * Pi) / (Overall Gear Ratio * 63360) var speedMPH = (engineRPM * tireDiameter * Math.PI) / (overallGearRatio * 63360); var speedKPH = speedMPH * 1.60934; resultDiv.innerHTML = "At " + engineRPM.toFixed(0) + " RPM, your vehicle speed is:" + speedMPH.toFixed(2) + " MPH (" + speedKPH.toFixed(2) + " KPH)"; }

Understanding Gear Ratios, RPM, and Vehicle Speed

The relationship between your engine's Revolutions Per Minute (RPM), your vehicle's gear ratios, and its road speed is fundamental to how any vehicle operates. This calculator helps you understand and predict your vehicle's speed at a given engine RPM, taking into account your transmission gear, final drive ratio, and tire size.

What is RPM?

RPM stands for Revolutions Per Minute. In the context of a vehicle, it typically refers to the rotational speed of the engine's crankshaft. Higher RPM means the engine is working harder and producing more power, but also consuming more fuel. Your tachometer displays this value.

What are Gear Ratios?

A gear ratio is the ratio of the number of teeth on two meshed gears, or the ratio of their rotational speeds. In a vehicle, there are typically two main gear ratios that affect road speed:

• Transmission Gear Ratio: This is the ratio within your gearbox (transmission) for each specific gear (1st, 2nd, 3rd, etc.). Lower gears (like 1st) have a higher numerical ratio, providing more torque for acceleration, while higher gears (like 5th or 6th) have a lower numerical ratio, allowing for higher speeds at lower engine RPM for fuel efficiency.
• Final Drive Ratio: Also known as the differential or axle ratio, this is the last gear reduction before power reaches the drive wheels. It's a fixed ratio for a given axle. A higher numerical final drive ratio (e.g., 4.10:1) provides quicker acceleration but lower top speed and higher RPM at cruising speeds. A lower numerical ratio (e.g., 3.08:1) offers better fuel economy and higher top speed, but slower acceleration.

The overall gear ratio is the product of the transmission gear ratio and the final drive ratio. This combined ratio determines how many times the engine crankshaft rotates for every single rotation of the drive wheels.

The Role of Tire Diameter

The tire diameter is crucial because it determines how much distance the vehicle covers with each rotation of the wheel. A larger diameter tire will cover more ground per revolution than a smaller one. This means that for the same engine RPM and gear ratio, a vehicle with larger tires will travel at a higher speed.

How They All Connect: The Formula

The calculator uses the following fundamental formula to determine vehicle speed:

Speed (MPH) = (Engine RPM × Tire Diameter × π) / (Overall Gear Ratio × 63360)

Where:

• Engine RPM is the engine's rotational speed.
• Tire Diameter is in inches.
• π (Pi) is approximately 3.14159.
• Overall Gear Ratio = Transmission Gear Ratio × Final Drive Ratio.
• 63360 is a conversion factor (12 inches/foot × 5280 feet/mile × 60 minutes/hour) to convert inches per minute to miles per hour.

Practical Examples

• Example 1: Cruising on the Highway
  If your engine is at 2500 RPM in 5th gear (ratio 0.85), with a 3.55 final drive ratio and 26-inch tires:
  Overall Gear Ratio = 0.85 × 3.55 = 3.0175
  Speed = (2500 × 26 × π) / (3.0175 × 63360) ≈ 67.2 MPH.
  This setup provides good fuel economy at highway speeds.
• Example 2: Performance Driving
  If you're in 3rd gear (ratio 1.50) at 6000 RPM, with a 4.10 final drive ratio and 24-inch tires:
  Overall Gear Ratio = 1.50 × 4.10 = 6.15
  Speed = (6000 × 24 × π) / (6.15 × 63360) ≈ 116.0 MPH.
  This demonstrates how lower gears and higher RPMs lead to significant speed, often used for acceleration or track driving.

By using this calculator, you can experiment with different gear ratios and tire sizes to understand their impact on your vehicle's performance, fuel efficiency, and top speed. It's a valuable tool for enthusiasts, mechanics, and anyone looking to optimize their vehicle's drivetrain.