1. What is a Car Speed Calculator by Gear Ratio?

Definition: This calculator estimates vehicle speed based on engine RPM, tire diameter, gear ratio, and axle ratio.

Purpose: It helps automotive enthusiasts and mechanics determine theoretical vehicle speed at specific RPMs for gear selection and performance tuning.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( S \) — Vehicle speed (mph)
• \( RPM \) — Engine revolutions per minute
• \( TD \) — Tire diameter (inches)
• \( GR \) — Transmission gear ratio (unitless)
• \( AR \) — Axle ratio (unitless)

Explanation: The formula converts engine rotation through the drivetrain ratios to wheel rotation, then calculates distance traveled per minute converted to mph.

3. Importance of Speed Calculation

Details: Accurate speed calculations help with gear selection, performance tuning, and ensuring legal speed capabilities for different driving conditions.

4. Using the Calculator

Tips: Enter engine RPM, tire diameter (measured from ground to top of tire), current gear ratio, and axle ratio. All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: How do I find my gear ratios?
A: Gear ratios are typically listed in your vehicle's manual or available from the manufacturer. Common ratios range from 2.5 to 4.5.

Q2: What's a typical axle ratio?
A: Axle ratios vary by vehicle but common values range from 2.73 to 4.10 for passenger vehicles, with higher numbers for trucks.

Q3: Why is tire diameter important?
A: Larger tires cover more ground per revolution, increasing speed at the same RPM compared to smaller tires.

Q4: Does this account for tire slip or torque converter slip?
A: No, this calculates theoretical speed. Actual speed may vary due to drivetrain losses, tire slip, or converter slip in automatics.

Q5: How accurate is this calculation?
A: It's theoretically accurate for perfect conditions. Real-world factors like tire wear, inflation, and load can affect actual speed.