Quarter Mile Gear Calculator

Quarter Mile Performance Factors:

Optimal gear ratio depends on:
- Engine RPM range
- Tire diameter
- Vehicle weight
- Desired trap speed

Max Power RPM:

RPM

Tire Diameter:

inches

Target Trap Speed:

mph

Transmission Gear Ratio:

Estimated Rear Gear Ratio:

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1. What is a Quarter Mile Gear Calculator?

Definition: This calculator estimates the optimal rear gear ratio for quarter mile racing based on your engine's power band, tire size, and target speed.

Purpose: Helps drag racers and performance enthusiasts select the best gear ratio to maximize acceleration over a quarter mile distance.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ \text{Gear Ratio} = \frac{\text{RPM} \times \text{Tire Diameter}}{336 \times \text{Trap Speed} \times \text{Transmission Ratio}} \]

Where:

RPM — Engine speed at peak power
Tire Diameter — Height of your rear tires in inches
Trap Speed — Desired speed at the finish line (mph)
Transmission Ratio — Gear ratio in your transmission's highest gear (1:1 for direct drive)

Explanation: This calculation ensures your engine reaches its power peak right at the finish line for maximum acceleration.

3. Importance of Gear Ratio Selection

Details: Proper gear ratio selection can shave seconds off your quarter mile time by keeping your engine in its optimal power band throughout the run.

4. Using the Calculator

Tips:

Use your dyno-tested peak power RPM
Measure actual tire diameter (not just sidewall numbers)
For manual transmissions, use your top gear ratio (often 1:1 for 4th or 5th gear)
For automatics, use your final drive ratio (typically 0.67-0.75 for overdrive)

5. Frequently Asked Questions (FAQ)

Q1: Why is tire diameter important?
A: Larger tires effectively create a taller gear ratio, while smaller tires create a shorter ratio. Actual rolling diameter affects how many revolutions your tires make per mile.

Q2: What if my car can't reach the target trap speed?
A: The calculation assumes your car has enough power. If you can't reach the speed, you may need more power or to adjust your expectations.

Q3: Should I gear for peak HP or peak torque?
A: Generally gear for peak horsepower RPM, as this gives the best average power through the run.

Q4: How much does vehicle weight affect the calculation?
A: Weight affects acceleration but not the optimal gear ratio calculation. Heavier cars may need more power to reach the same trap speed.

Q5: What's a typical gear ratio for quarter mile cars?
A: Street cars typically use 3.55-4.10 ratios, while dedicated drag cars may use 4.56-5.13 or higher, depending on tire size and engine RPM.