Gear Ratio Speed Calculator MPH RPM

Vehicle Speed Formula:

\[ \text{MPH} = \frac{\text{RPM} \times \text{tire\_diameter} \times \pi \times \text{gear\_ratio} \times \text{final\_drive}}{336 \times \text{transmission\_ratio}} \]

RPM:

RPM

Tire Diameter:

inches

Gear Ratio:

Final Drive Ratio:

Transmission Ratio:

Vehicle Speed (MPH):

Unit Converter ▲

Unit Converter ▼

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1. What is a Gear Ratio Speed Calculator?

Definition: This calculator estimates vehicle speed in miles per hour (MPH) based on engine RPM, tire size, and gear ratios.

Purpose: It helps automotive enthusiasts and mechanics determine speed at specific RPM points for gear selection and performance tuning.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ \text{MPH} = \frac{\text{RPM} \times \text{tire\_diameter} \times \pi \times \text{gear\_ratio} \times \text{final\_drive}}{336 \times \text{transmission\_ratio}} \]

Where:

\( \text{MPH} \) — Vehicle speed (miles per hour)
\( \text{RPM} \) — Engine revolutions per minute
\( \text{tire\_diameter} \) — Tire diameter in inches
\( \text{gear\_ratio} \) — Selected gear ratio
\( \text{final\_drive} \) — Differential/axle ratio
\( \text{transmission\_ratio} \) — Transmission ratio (1:1 for direct drive)
\( 336 \) — Conversion constant (combines unit conversions)

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

3. Importance of Gear Ratio Calculations

Details: Proper gear ratio selection affects acceleration, top speed, fuel economy, and engine longevity by keeping RPM in optimal ranges.

4. Using the Calculator

Tips: Enter RPM, tire diameter (measured from ground to top of tire), gear ratio, final drive ratio, and transmission ratio (default 1 for direct drive).

5. Frequently Asked Questions (FAQ)

Q1: How do I measure tire diameter accurately?
A: Measure from ground to top of tire with proper inflation and vehicle weight, or use manufacturer specs.

Q2: What if I have an overdrive transmission?
A: Use the overdrive ratio (typically 0.6-0.8) for the transmission ratio when in overdrive.

Q3: Why is 336 the conversion constant?
A: It combines conversions from inches to miles (63360), minutes to hours (60), and simplifies the π/2 calculation.

Q4: How does this help with gear selection?
A: You can calculate speed at redline to ensure proper gear ratios for your performance goals.

Q5: Can I use this for motorcycle calculations?
A: Yes, the formula works for any wheeled vehicle - just input the correct values for your bike.