Which Quarter-Mile Formula Is Most Accurate?
Huntington vs. Fox vs. Hale — the three classic equations tested against real time slips, and which to trust for your car.
Every quarter-mile calculator on the internet runs one of three empirical formulas, all cube-root laws of power-to-weight fitted to real time-slip data decades apart. They agree within a couple of tenths for ordinary cars — the interesting question is where each one drifts, and which output you should believe.
The Three Formulas
| Formula | ET constant | MPH constant | Origin |
|---|---|---|---|
| Huntington (1950s) | 6.290 | 224 | Curve fit of 1950s Hot Rod era time slips |
| Fox | 6.269 | 230 | Geoffrey Fox's physics analysis (Am. J. Phys. 41, 311, 1973) and his later empirical refit |
| Hale (1980s) | 5.825 | 234 | Patrick Hale's refit for later tires and aero (Quarter Mile Math) |
All use ET = C × (Weight ÷ HP)⅓ and MPH = C × (HP ÷ Weight)⅓. Huntington's and Fox's ET constants barely differ (0.3%), but Hale's 5.825 is 7.4% lower — a noticeably quicker prediction. The MPH constants spread from 224 to 234, a 4.5% difference in predicted trap speed that cubes into a ~13% difference when you invert the formula to estimate horsepower from a time slip.
Tested Against Real Cars
Using published specs and typical tested results (driver included):
| Car | Tested | Huntington | Fox | Hale |
|---|---|---|---|---|
| MX-5 (181 hp / 2,520 lb) | 14.5 @ 95 | 15.1 @ 93 | 15.1 @ 96 | 14.0 @ 97 |
| Mustang GT (480 / 4,010) | 12.4 @ 112 | 12.8 @ 110 | 12.7 @ 113 | 11.8 @ 115 |
| Corvette Z06 (670 / 3,610) | 10.6 @ 131 | 11.0 @ 128 | 11.0 @ 131 | 10.2 @ 133 |
| Model 3 Perf. (510 / 4,230) | 11.7 @ 115 | 12.7 @ 111 | 12.7 @ 114 | 11.8 @ 116 |
The pattern for the three gas cars: Huntington and Fox ETs run 0.4–0.6 s slow (launch control and sticky tires beat 1950s-calibrated launches), while Hale's ET runs 0.4–0.6 s quick — the tested figure sits between them. On trap speed, Huntington reads 2–3 MPH low, Fox lands within about 1 MPH and Hale 2–3 MPH high. The EV is the outlier: about a second quicker in ET than Huntington or Fox predict (instant torque, AWD launch), landing close to Hale's optimistic figure. Published Model 3 Performance runs vary widely (roughly 10.9–11.8 s), so treat that row as indicative. Trap speed obeys physics; ET rewards the launch.
So Which Should You Use?
- Predicting your ET/MPH from horsepower: our quarter-mile calculator pairs Huntington's conservative ET constant (6.290) with Hale's trap-speed constant (234). Expect a good launch to beat the predicted ET by a few tenths; Hale's 5.825 gives the optimistic bound.
- Estimating horsepower from a time slip: use the trap-speed inversion, HP = W × (MPH ÷ 234)³ — see trap speed HP. ET-based estimates absorb wheelspin and driver error.
- EVs and AWD launch monsters: trust trap-speed math; Huntington- and Fox-based ETs over-predict them by about a second.
- Corrections: density altitude shifts everything — a hot, high track adds tenths (see altitude loss).
Frequently Asked Questions
One read via trap speed, using Fox's (230) or Hale's (234) constant — both land within about 1–3 MPH for modern gas cars. ET predictions are always softer because launch quality isn't in the equation; the real ET usually falls between Huntington's 6.290 and Hale's 5.825 predictions.
Launch control, sticky tires and AWD get you through the formula's weakest region — the first 60 feet — faster than the historical data it was fitted on. Underrated factory power figures help too.
Trap speed yes, ET only loosely. EVs launch far harder than the Huntington and Fox formulas assume, running about a second quicker in ET while trapping close to where power-to-weight predicts.