Calculating Gear Ratios And Speed

Formulas:

\[ GR = \frac{RPM_{out}}{RPM_{in}} \] \[ S = \frac{RPM \times \pi \times TD}{GR \times AR \times 1056} \]

Output RPM:

RPM

Input RPM:

RPM

Engine RPM:

RPM

Tire Diameter:

inches

Axle Ratio:

Gear Ratio:

Speed:

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is a Gear Ratio and Speed Calculator?

Definition: This calculator determines the gear ratio between input and output RPM, and calculates vehicle speed based on engine RPM, tire diameter, and axle ratio.

Purpose: It helps automotive professionals and enthusiasts understand their vehicle's gearing and predict speed at different RPM levels.

2. How Does the Calculator Work?

The calculator uses two formulas:

\[ GR = \frac{RPM_{out}}{RPM_{in}} \] \[ S = \frac{RPM \times \pi \times TD}{GR \times AR \times 1056} \]

Where:

\( GR \) — Gear ratio (unitless)
\( RPM_{out} \) — Output shaft revolutions per minute
\( RPM_{in} \) — Input shaft revolutions per minute
\( S \) — Speed (mph)
\( RPM \) — Engine revolutions per minute
\( TD \) — Tire diameter (inches)
\( AR \) — Axle ratio (unitless)

Explanation: The gear ratio shows how many times the input shaft must rotate to make the output shaft rotate once. The speed formula converts engine RPM to vehicle speed considering all mechanical ratios.

3. Importance of Gear Ratio and Speed Calculation

Details: Proper gearing calculations ensure optimal vehicle performance, fuel efficiency, and help in transmission and differential selection.

4. Using the Calculator

Tips: Enter the output and input RPM for gear ratio calculation, then engine RPM, tire diameter, and axle ratio (default 3.73) for speed calculation. All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical axle ratio?
A: Common axle ratios range from 3.08 (for fuel efficiency) to 4.10 (for acceleration), with 3.73 being a popular all-around choice.

Q2: How do I measure tire diameter?
A: Measure from ground to top of tire or use manufacturer specifications. The formula is: (wheel diameter) + 2*(sidewall height).

Q3: What does the 1056 constant represent?
A: It's a conversion factor that accounts for units (inches to miles and minutes to hours) and the π calculation.

Q4: How does gear ratio affect speed?
A: Higher gear ratios (numerically lower) allow higher speeds at lower RPM, while lower ratios (numerically higher) provide better acceleration but lower top speed.

Q5: Can I use this for transmission gear calculations?
A: Yes, input/output RPM can be from any gear set (transmission gears, transfer case, differential, etc.).