1. What is a Speed Calculator (MPH from RPM)?

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

Purpose: It helps automotive professionals and enthusiasts determine vehicle speed at specific RPMs for gear tuning and performance analysis.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( S \) — Speed (miles per hour)
• \( RPM \) — Engine revolutions per minute
• \( TD \) — Tire diameter (inches)
• \( GR \) — Gear ratio (unitless)
• \( AR \) — Axle ratio (unitless)
• \( 1056 \) — Conversion constant (combines inches to miles and minutes to hours)

Explanation: The formula calculates how far the vehicle travels per minute based on tire rotation, then converts this to miles per hour.

3. Importance of Speed Calculation

Details: Accurate speed calculation helps in gear selection, fuel efficiency optimization, and ensuring speedometer accuracy.

4. Using the Calculator

Tips: Enter the engine RPM, measured tire diameter (from ground to top of tire × 2), current gear ratio (typically 1:1 in high gear), and vehicle's axle ratio.

5. Frequently Asked Questions (FAQ)

Q1: Why is tire diameter important?
A: Larger tires cover more distance per revolution, increasing speed at the same RPM compared to smaller tires.

Q2: What if I don't know my axle ratio?
A: Check your vehicle's manual or look for a tag on the differential. Common ratios range from 3.08 to 4.10.

Q3: How does gear ratio affect speed?
A: Lower gear ratios (numerically higher) provide more torque but lower top speed at a given RPM.

Q4: What's the 1056 constant?
A: It combines conversion from inches to miles (63360) and minutes to hours (60) → 63360/60 = 1056.

Q5: Does this account for tire slippage?
A: No, this calculates theoretical speed. Actual speed may vary due to tire slip, load, or inflation.