Example Data Table
| Input | Example Value | Meaning |
|---|---|---|
| Tire Diameter | 28 inches | Actual loaded tire diameter |
| Shift RPM | 7500 rpm | Engine speed used for gear speed |
| Final Drive | 4.10 | Rear axle or final gear ratio |
| Gear Ratios | 2.48, 1.48, 1.00 | Transmission ratios for each racing gear |
| Slip | 5% | Estimated converter or clutch slip |
Formula Used
Overall ratio = gear ratio × final drive ratio.
Slip factor = 1 + slip percentage ÷ 100.
Speed = RPM × tire diameter ÷ overall ratio ÷ 336 ÷ slip factor.
Wheel torque = engine torque × overall ratio × drivetrain efficiency.
Shift recovery RPM = shift RPM × next gear ratio ÷ current gear ratio.
Recommended final drive = desired trap RPM × tire diameter ÷ target speed ÷ top gear ÷ 336 ÷ slip factor.
How to Use This Calculator
- Enter the real tire diameter in inches.
- Add your planned shift rpm or maximum engine rpm.
- Enter final drive, slip, torque, and drivetrain efficiency.
- Add each transmission gear ratio. Use zero for unused gears.
- Enter the target trap speed and finish line safety margin.
- Press calculate to view speed, torque, shift drop, and trap rpm.
- Use CSV or PDF buttons to save the current gearing report.
Drag Racing Gear Planning Guide
Why gearing matters
Drag racing gearing is a balance between acceleration, traction, and finish line rpm. A short overall ratio multiplies torque hard. It can launch the car quickly. It can also cause spin, early shifts, and wasted power. A tall overall ratio calms the launch. It may lower engine speed too much after each shift. The best setup keeps the engine near its useful power band while the car moves through each gear.
What the calculator checks
This calculator estimates vehicle speed at a selected engine speed for every entered gear. It also estimates wheel torque after final drive multiplication and drivetrain loss. The shift drop value shows the rpm after changing into the next gear at the chosen shift rpm. Smaller drops usually keep the engine happier. Large drops can pull the engine below its strong range. The target trap rpm field helps compare your final drive against the expected finish line speed.
Reading the results
Start by looking at the last racing gear you expect to use. Its speed should cover your planned trap speed with a small safety margin. Then review first gear wheel torque. Very high torque may need tire, suspension, or launch control changes. Next, compare shift drops. The rpm after each shift should remain above the engine speed where power begins to fade. The calculator does not replace track data. It gives a clean baseline before testing.
Practical tuning tips
Use real tire diameter, not only sidewall size. Slicks can grow at speed. Radials usually grow less. Converter slip also changes during a pass. Enter a realistic slip percentage for your setup. Automatic cars often need more margin than clutch cars. Weather, track prep, vehicle weight, and power delivery all change the final choice. Make one change at a time. Record every pass. Use the export buttons to keep notes with each ratio plan. Good gearing should improve consistency, not only one lucky run. When the numbers agree with data, tuning becomes calmer and faster.
Safety and limits
Use the output as an estimate. Driveshaft speed limits, tire ratings, clutch heat, converter behavior, and rule book requirements must be checked separately before any race pass at full power.
FAQs
1. What does this drag racing gear calculator estimate?
It estimates speed per gear, wheel torque, shift recovery rpm, target trap rpm, and a suggested final drive ratio based on your entered tire, rpm, gear, and drivetrain data.
2. Why is tire diameter important?
Tire diameter changes how far the car travels per wheel rotation. A taller tire raises speed for the same rpm. A shorter tire increases effective gearing.
3. What is converter slip?
Converter slip is the difference between engine speed and drivetrain speed. More slip reduces actual road speed at a given engine rpm, especially in automatic racing setups.
4. How do I enter unused gears?
Enter zero for any unused gear field. The calculator only processes gear ratios greater than zero and builds shift recovery values from the active gears.
5. What is shift recovery rpm?
Shift recovery rpm is the estimated engine rpm after a gear change. It helps show whether the next gear keeps the engine inside its useful power band.
6. Is the recommended final drive always correct?
No. It is a mathematical estimate. Track grip, tire growth, power curve, converter behavior, and racing rules can change the best final drive choice.
7. Should I use engine torque or wheel torque?
Enter engine torque. The calculator multiplies it by gear ratio, final drive, and drivetrain efficiency to estimate wheel torque for each gear.
8. Can I save my gearing results?
Yes. Use the CSV button for spreadsheet data. Use the PDF button for a simple report that includes inputs, formulas, and gear results.