Calculator
Formula Used
This calculator estimates quarter-mile performance from eighth-mile data using multipliers:
- Estimated 1/4-mile ET:
ET¼ = ET⅛ × kET - Estimated 1/4-mile Speed:
V¼ = V⅛ × kV
To show uncertainty, it also computes a range:
ET¼(low) = ET⅛ × kET(low),ET¼(high) = ET⅛ × kET(high)V¼(low) = V⅛ × kV(low),V¼(high) = V⅛ × kV(high)
Because cars accelerate differently past the 1/8-mile, multipliers vary by traction, gearing, power curve, aero, and shift strategy.
How to Use This Calculator
- Enter your 1/8-mile ET from the time slip.
- If available, enter 1/8-mile trap speed and pick units.
- Select a conversion method that matches your setup.
- Optionally add a small correction (%) and launch bias.
- Click Calculate, then download CSV or PDF if needed.
For best accuracy, compare estimates to a few real 1/4-mile passes and tune multipliers or correction values until they match your track and conditions.
Example Data
Use this quick sample to see how the calculator behaves:
- 1/8-mile ET: 7.85 s
- 1/8-mile speed: 90.5 mph
- Method: Generic (balanced)
- Launch bias: 0.03 s
- Density altitude: 1500 ft
Example Data Table
| 1/8 ET (s) | 1/8 Speed (mph) | Method | Est. 1/4 ET (s) | Est. 1/4 Speed (mph) |
|---|---|---|---|---|
| 7.85 | 90.5 | Generic | 12.40 | 113.1 |
| 8.60 | 82.0 | Street | 13.76 | 100.9 |
| 6.95 | 100.0 | Slick | 10.91 | 127.0 |
| 7.30 | 95.0 | Generic | 11.53 | 118.8 |
1) What the conversion is really doing
The tool scales your 660‑ft results to predict 1320‑ft performance. A common baseline is ET multiplier 1.56–1.62 and speed multiplier 1.21–1.29. Faster cars often gain more speed in the second half, while traction‑limited cars may lose time after the eighth.
2) Why ET and speed multipliers differ
Elapsed time depends heavily on launch, shifts, and early acceleration. Trap speed reflects power and how well the car keeps pulling past mid‑track. That is why the calculator uses separate factors kET and kV instead of one universal number.
3) Using the uncertainty range
Along with a single estimate, the calculator shows a low‑to‑high range. Treat it like a confidence band: if your car is consistent, your real quarter should land near the middle. If gearing changes or you miss a shift, expect results near the edges. A tight range is typical when your multipliers are well tuned. If your past quarters vary by 0.2–0.4 seconds, keep the wider band and log more runs.
4) Correction percent for quick tuning
The correction field applies a small percent change to both outputs. For example, +1.5% on a 12.40‑second estimate becomes about 12.59 seconds. Use it to match your track, tire, and repeatable setup without rewriting multipliers.
5) Launch bias for traction differences
Launch bias adds seconds only to the estimated quarter ET. It is helpful when your eighth‑mile looks strong, but the car fades later due to heat soak, spin on the 1–2 shift, or soft suspension. Typical tuning values are 0.00–0.10 s.
6) Speed units and slip accuracy
You can enter mph or km/h, and the calculator converts internally for consistent math. Use the speed from the time slip, not the dashboard. Even a 1 mph error at the eighth can move the quarter estimate by 1–2 mph depending on multiplier choice.
7) Density altitude as a context clue
Higher density altitude usually slows ET and reduces trap speed. The tool shows a rough suggestion so you can decide whether to apply a small negative or positive correction. Always validate with real passes, because aero, tune, and boost control change the outcome.
FAQs
1) Do I need both 1/8 ET and 1/8 speed?
No. Enter either value to get an estimate. Providing both gives a more complete picture because you will see both predicted quarter ET and predicted quarter trap speed.
2) Which method should I choose?
Start with Generic. If you have strong traction and hard launches, try Drag radial / slick. For typical street tires or heat‑soaked runs, Street / bracket may fit better. Compare with a real quarter pass to confirm.
3) Why is my real quarter ET slower than the estimate?
Common causes include wheelspin after the eighth, missed shifts, lifting early, poor aero at high speed, and rising intake temperatures. Add a small launch bias or choose a more conservative multiplier set.
4) Can I make it match my local track?
Yes. Use the correction percent first, then switch to Custom multipliers if you want a permanent profile. Save your preferred values and reuse them for consistent comparisons across different weather.
5) Is the trap speed estimate always reliable?
It is a useful guideline, not a guarantee. Power delivery, gear changes, and aero affect the second half differently than the first. If you have turbo lag or strong top‑end power, fine‑tune the speed multiplier.
6) How should I use the CSV and PDF exports?
Export CSV to log runs and compare setups over time. Export PDF to share a clean summary with your tuner or friends. Include your method, correction, and bias so others can reproduce your estimate.