1. What is a Rear Gear Calculator for 1/8th Mile?

Definition: This calculator determines the optimal rear gear ratio for 1/8th mile drag racing based on engine RPM, tire diameter, trap speed, and final drive ratio.

Purpose: It helps racers and performance enthusiasts optimize their vehicle's gearing for maximum acceleration in 1/8th mile competitions.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( GR \) — Gear ratio (factor)
• \( RPM \) — Engine revolutions per minute
• \( TD \) — Tire diameter (inches)
• \( MPH \) — Trap speed at finish line (miles per hour)
• \( FD \) — Final drive ratio (factor)

Explanation: The formula calculates the rear gear ratio needed to achieve the observed trap speed at the given RPM, accounting for tire size and transmission gearing.

3. Importance of Gear Ratio Calculation

Details: Proper gear ratio selection ensures the engine operates in its optimal power band throughout the 1/8th mile run, maximizing acceleration and performance.

4. Using the Calculator

Tips: Enter the engine RPM at finish line, tire diameter in inches, trap speed in MPH, and final drive ratio (default 1.0). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: Why is the constant 336 used in the formula?
A: The 336 factor converts units (miles to inches and hours to minutes) to yield a proper gear ratio.

Q2: What's a typical final drive ratio?
A: Most vehicles use 1.0 for direct drive (1:1 ratio in high gear), but automatic transmissions may have different ratios.

Q3: How do I measure tire diameter accurately?
A: Measure from ground to top of tire under race weight, or use manufacturer specs for unloaded diameter.

Q4: Should I use peak RPM or shift point RPM?
A: Use the RPM at the finish line (trap speed), which may be different from your shift points.

Q5: How does this differ from 1/4 mile calculations?
A: The formula is the same, but 1/8th mile gearing typically favors quicker acceleration over top speed.