1/8 Mile Gear Ratio Calculator Drag Racing

Gear Ratio Formula:

\[ GR = \frac{RPM \times TD}{MPH \times FD \times 336} \]

Engine RPM:

rev/min

Tire Diameter:

inches

Trap Speed:

mi/h

Final Drive Ratio:

Gear Ratio:

Unit Converter ▲

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1. What is a 1/8 Mile Gear Ratio Calculator?

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

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

2. How Does the Calculator Work?

The calculator uses the formula:

\[ GR = \frac{RPM \times TD}{MPH \times FD \times 336} \]

Where:

\( GR \) — Gear Ratio (factor)
\( RPM \) — Engine revolutions per minute
\( TD \) — Tire diameter (inches)
\( MPH \) — Trap speed at 1/8 mile (miles per hour)
\( FD \) — Final drive ratio (factor)
336 — Constant for unit conversion

Explanation: The formula calculates the gear ratio needed to achieve a specific trap speed at a given RPM, accounting for tire size and final drive ratio.

3. Importance of Gear Ratio in Drag Racing

Details: Proper gear ratio selection ensures the engine operates in its optimal power band throughout the race, maximizing acceleration and minimizing ET.

4. Using the Calculator

Tips: Enter your engine's maximum RPM, tire diameter in inches, target trap speed in MPH, and final drive ratio (default 1.0). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What's the 336 constant in the formula?
A: It's a conversion factor that accounts for units (miles to inches and hours to minutes) in the calculation.

Q2: How do I measure tire diameter accurately?
A: Measure from the ground to the top of the tire with the vehicle's weight on it, or use manufacturer specs for unloaded diameter.

Q3: Should I use peak RPM or shift RPM?
A: Use the RPM at which you'll be crossing the finish line (typically slightly below your shift point for most setups).

Q4: What if I have a torque converter or clutch slip?
A: You may need to adjust your RPM input to account for any slippage in your drivetrain.

Q5: How does this differ from 1/4 mile calculations?
A: The formula is similar, but 1/8-mile calculations typically use different target RPMs and speeds since the power band usage differs.