ET is elapsed time over 1,320 ft (402.336 m), excluding reaction time. Trap speed is speed measured near the finish; race weight includes the car, driver, fuel and equipment. The default Fox model uses ET = 6.269 × ∛(lb/hp) and MPH = 230 × ∛(hp/lb), with pounds and mechanical engine horsepower as base units. Trap speed reflects sustained power more consistently; ET is strongly affected by the launch.
Let W be race weight in pounds and P be mechanical engine hp. ET = A × ∛(W/P); MPH = B × ∛(P/W). Reverse them as P = W × (A/ET)³ or P = W × (MPH/B)³.
| Model | A (ET) | B (MPH) | Interpretation |
|---|---|---|---|
| Fox | 6.269 | 230 | Recommended street-car starting point |
| Hale | 5.825 | 234 | Idealized, well-prepared race-car estimate |
| Huntington | 6.290 | 224 | Classic engine/clutch-power estimate |
Lucius’s formula history documents these coefficients, including Fox’s later empirical constants and Huntington’s original clutch-power basis. Fox is a useful default, not a universal accuracy winner. None is calibrated directly to chassis-dyno WHP.
Our distance-scaling assumption is constant power without drag or launch losses: v ∝ distance^(1/3), t ∝ distance^(2/3). Thus eighth-mile ET = quarter-mile ET × (1/2)^(2/3) ≈ ET × 0.6299605; eighth-mile speed = quarter-mile speed × ∛(1/2) ≈ speed × 0.7937005. This page’s extrapolation is not a measured split or part of the historical quarter-mile calibration. Distance is 660 ft (201.168 m).
Using Fox, W/P = 3,500/400 = 8.75 lb/hp and ∛8.75 = 2.060643. ET = 6.269 × 2.060643 = 12.92 s. Trap speed = 230/2.060643 = 111.6 mph; × 1.609344 = 179.6 km/h. No drivetrain conversion is applied. Hale gives 12.00 s at 113.6 mph; Huntington gives 12.96 s at 108.7 mph.
Using Fox in measured trap-speed mode: P = 3,200 × (110/230)³ = 3,200 × 0.109394 ≈ 350.1 crank hp. If a 15% drivetrain loss is assumed, the corresponding wheel estimate is 350.1 × 0.85 ≈ 297.6 WHP. That conversion does not turn a speed-based estimate into a chassis-dyno measurement.
Sample declared before calculating errors: one light roadster (Miata), V8 coupe (Mustang GT), and turbocharged front-drive hatchback (Civic Type R), all factory-stock manuals. These three examples cover different layouts; they are not a statistically representative fleet. Linked Car and Driver instrumented tests supply curb weights, manufacturer crank ratings and published performance. No wheel-dyno figures or EVs are mixed in.
For reproducibility, estimated race weight = published curb weight + 200 lb for driver and test equipment. This fixed allowance is our assumption, not a reported test-day load. No additional fuel weight is added. Use the published results as reported; actual staging, weather corrections and test loads are not reconstructed.
| Car / original test | Curb + allowance = race lb | Crank hp | Published | Fox | Hale | Huntington |
|---|---|---|---|---|---|---|
| 2019 Mazda MX-5 Miata Club, 6MT | 2,345 + 200 = 2,545 | 181 | 14.50 / 95.0 | 15.13 / 95.3 | 14.06 / 96.9 | 15.18 / 92.8 |
| 2018 Ford Mustang GT, 6MT | 3,878 + 200 = 4,078 | 460 | 12.60 / 115.0 | 12.97 / 111.1 | 12.06 / 113.1 | 13.02 / 108.2 |
| 2017 Honda Civic Type R, 6MT | 3,111 + 200 = 3,311 | 306 | 13.50 / 108.0 | 13.87 / 104.0 | 12.88 / 105.8 | 13.91 / 101.3 |
| Model | ET MAE | MPH MAE |
|---|---|---|
| Fox | 0.457 s | 2.725 mph |
| Hale | 0.534 s | 2.030 mph |
| Huntington | 0.504 s | 5.229 mph |
Our calculation: MAE = sum of |prediction − published value| ÷ 3, using unrounded predictions. Coefficients were not fitted to these cars. Hale predicts a quicker ET than published for all three; this small check supports caution about ideal-pass estimates, not a universal correction factor. Model rankings can change with the sample.
Car and Driver’s test methodology describes weather corrections and street-surface testing. Its rollout explanation documents a 2019 procedure change; these original tests predate that change. Magazine figures and a current drag-strip slip need not use identical timing conventions.
A weak launch can cost ET even when trap speed is close to predicted. If both measures are worse, power delivery, shifts, weather or resistance may contribute. A two-number slip cannot isolate a mechanical fault. Gearing, tire grip, aerodynamics and the full power curve are not simulated.
At 3,500 lb race weight, Fox implies approximately 862, 648, and 499 crank hp for 10, 11, and 12 seconds respectively. Choose Required HP for a target and enter your own weight and ET. These are power estimates, not guarantees that the tires, drivetrain or chassis can deliver the pass.
Trap speed usually depends less on launch quality, so start with Estimate HP from measured trap speed. Enter ET as well to compare both implied figures. Neither is a dyno measurement: wind, aerodynamics, shifts and the power curve still matter.
A slower first 60 feet adds time directly and can also reduce speed through the rest of the run. There is no universal multiplier between a 60-foot improvement and quarter-mile ET. Compare repeat slips at the same track; this calculator does not model a 60-foot split.
Yes. The ET clock begins when the front tire clears the stage beam, after some initial movement. Staging depth and tire geometry change rollout. Reaction time is separate. Do not add or subtract a fixed rollout correction here; compare timings obtained with the same convention.
Higher density altitude means less dense air and generally less oxygen for combustion, especially affecting naturally aspirated engines. Boost control can partly compensate; aerodynamic drag also changes. No weather correction is applied here. Use power appropriate to the conditions and compare passes in similar weather.
Only as a rough, unvalidated extrapolation. Motorcycle rider weight, wheelies and aerodynamics differ greatly; EV power varies with battery state, temperature and speed. The validation sample contains only combustion cars. These models cannot predict those effects.
Yes. Include the driver, fuel, fluids, safety equipment and everything carried during the run. A curb weight alone understates race weight.
Select your basis in Advanced. All supported historical models use engine/clutch power. Wheel inputs are divided by one minus the assumed drivetrain-loss fraction before calculation; reverse results are multiplied by that factor to estimate wheel power. Crank inputs are not reduced. A loss percentage is an assumption, not a dyno correction.
Enable approximate eighth-mile results in Advanced. The page scales quarter-mile ET by 0.6299605 and speed by 0.7937005, using ideal constant-power distance scaling. This is not a simulated or validated 660-foot split. Reverse and target inputs must still be quarter-mile measurements.
No. Each is an independent power-to-weight correlation. The model spread is a comparison of formulas, not a confidence interval. A slower ET with similar trap speed can indicate launch or traction losses, but cannot diagnose them alone.
No. The calculator runs locally and does not upload, store or attach the entered values to analytics events.
These correlations are estimates, not a performance guarantee or vehicle setup certification. They do not simulate traction, shifts, drag, weather or changing power. The assumed loss conversion is not a drivetrain model. Reverse estimates inherit all these limitations.
Test only at a sanctioned motorsport facility under its technical and safety rules. Never test acceleration or top speed on public roads.
Maintained by Starlight Robotics. Updated September 24, 2026. No named independent technical review is claimed.
Calculation checks cover forward/reverse round trips for all models, unit conversions, drivetrain assumptions, target estimates and the published-test error table. These verify implementation, not real-world predictive accuracy.
Revision history: September 24, 2026 — added reverse and target modes, model comparisons, power-basis conversion, approximate eighth-mile results, examples and a reproducible validation sample. August 2, 2026 — original Hale forward calculator.