Calibrate against paired slips
Start with complete time slips that include both distances for the same vehicle. Divide quarter-mile ET by eighth-mile ET for each representative pass. Compare those observed factors and avoid carrying them across substantially different power, gearing or conditions. The worksheet exposes the factor so its assumption is visible.
Estimated quarter-mile ET = eighth-mile ET × selected factorFollow the worked comparison below for the input units and assumptions.
Work through an example
At an eighth-mile ET of 8.00 seconds, a selected factor of 1.57 gives 12.56 seconds. A factor of 1.60 gives 12.80 seconds. That 0.24-second sensitivity matters more than displaying extra decimal places.
Calculate a factor from paired time slips
For each representative pass, divide quarter-mile ET by eighth-mile ET. Keep paired values from the same pass together, then examine the spread of factors. A factor from a different vehicle or power setup can hide a different second-half acceleration pattern.
Show a range when the factor is uncertain
Multiply the same eighth-mile ET by a low and high plausible factor from your records. For 8.00 seconds, factors of 1.57 and 1.60 span 12.56 to 12.80 seconds. A range communicates the assumption more clearly than a single hundredth-second prediction.
Show conversion-factor uncertainty as a range
Fix eighth-mile ET at 8.00 seconds and vary the factor rather than hiding it in the calculation. This example demonstrates a heuristic sensitivity comparison. It does not describe a universal relationship between the two distances or predict the second half of a particular run.
Estimated quarter-mile ET = eighth-mile ET × selected factor- Collect paired eighth-mile and quarter-mile ETs from the same representative passes.
- Divide each quarter-mile ET by its paired eighth-mile ET to inspect the observed factors.
- Use a representative range rather than assuming the default is appropriate for another vehicle.
- Repeat the comparison after a substantial gearing or power change, because second-half acceleration behavior may change.
| Eighth-mile ET (s) | Selected factor | Estimated quarter-mile ET (s) |
|---|---|---|
| 8 | 1.54 | 12.32 |
| 8 | 1.57 | 12.56 |
| 8 | 1.6 | 12.8 |
Changing the factor from 1.57 to 1.60 moves the result from 12.56 to 12.80 seconds even though the input pass is unchanged. A provisional range exposes that sensitivity more clearly than a single result displayed to many decimal places.
Check the inputs before interpreting a difference
Do not double ET simply because the distance doubles: the vehicle is already moving at the eighth-mile mark. Also do not treat this ET multiplier as a speed conversion or a calibrated bracket-racing dial-in.
Before you apply the result
Doubling distance does not require doubling time because the vehicle is already moving at the eighth-mile mark. The default 1.57 is an illustrative heuristic, not a verified factor for your car and not a guaranteed dial-in.
Try the calculation
Enter your own values in the related calculators. Compare one input at a time and keep units and operating conditions visible with the result.
Frequently asked questions
Why not double the eighth-mile time?
The vehicle is already moving at the eighth-mile mark. The second half is not a second standing-start run.
Is a 1.57 conversion factor universal?
No. It is an illustrative default. Use paired slips to examine the factor for your own vehicle and conditions.
Does this convert eighth-mile MPH to quarter-mile MPH?
No. This worksheet multiplies ET by an entered factor; it does not model the complete speed history.
How do I estimate a conversion factor from my own time slips?
For each complete pass, divide quarter-mile ET by eighth-mile ET. Examine several representative paired passes and keep the factors tied to that vehicle and setup.
Reference and method
Hale coefficient model: ET constant 5.825 and speed constant 234. Lucius documents the formulas and compares them with road-test data. These are empirical relationships, not physical laws or calibrated vehicle predictions.
Jeff Lucius / Stealth 316 — Quarter-mile formulas and original regression analysis ↗These explanations and examples describe the documented model. No manufacturer testing, named expert authorship or independent professional review is claimed. Read the method policy or report a correction.