1. What is a Speed Calculator by Gear Ratio?

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

Purpose: It helps automotive enthusiasts and mechanics determine theoretical vehicle speed 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 accounts for engine speed, tire circumference, and the combined reduction of transmission and axle gearing.

3. Importance of Speed Calculation

Details: Accurate speed calculation helps with gear selection, performance tuning, and ensuring legal speed limits are maintained.

4. Using the Calculator

Tips: Enter engine RPM, tire diameter in inches, gear ratio (e.g., 1.0 for direct drive), and axle ratio (default 3.73). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What's the 1056 constant in the formula?
A: It combines unit conversions (inches to miles, minutes to hours) and π/12 for tire circumference.

Q2: How do I measure tire diameter?
A: Measure from ground to top of tire (loaded) or use manufacturer specs. For more accuracy, use (tire width * aspect ratio / 1270) + wheel diameter.

Q3: Where do I find gear ratios?
A: Check your vehicle's manual or manufacturer specifications for each gear's ratio.

Q4: Why is actual speed sometimes different?
A: This calculates theoretical speed. Actual speed may vary due to tire slip, converter slip (automatics), or load conditions.

Q5: How does this help with gear selection?
A: You can compare different gear ratios to find optimal performance or fuel economy for your RPM range.