Run a quarter mile calculatoragainst ten showroom cars and the trap speed lands within about 1 mph — while the elapsed time comes out 0.44 seconds off on average, and slow in eight of the ten. Those two error bars are wildly different, and the gap between them is the most useful thing on your timeslip. One number is measuring your engine. The other is measuring your right foot, your tyres and the first 60 feet of track.
Two Formulas, and They Disagree by Half a Second
Every quarter-mile predictor you'll find online runs one of two cube-root equations, and they don't return the same answer. Both take race weight in pounds and flywheel horsepower; both assume nothing about your engine beyond its peak power figure.
| Formula | ET | Trap speed | 3,880 lb / 480 hp |
|---|---|---|---|
| Patrick Hale | 5.825 × (W÷HP)^⅓ | 234 × (HP÷W)^⅓ | 11.69 s @ 116.6 mph |
| Roger Huntington | 6.290 × (W÷HP)^⅓ | 224 × (HP÷W)^⅓ | 12.63 s @ 111.6 mph |
Nearly a full second apart on the same car. Neither is wrong — they were fitted to different decades. Huntington was an automotive journalist working from 1960s muscle-car timeslips, back when a 400-horsepower car left the line on bias-ply tyres barely wider than a modern spare. Hale's constants came out of computer-modelled data with modern rubber underneath. Our calculator runs Hale as the baseline because that's what current tyres actually deliver, then shows you where Huntington would put you. Real runs almost always land between the two.
Notice the exponent. Both use a cube root, which means power and weight have a compressed effect on ET. Double your horsepower and you don't halve your time — you cut it by 21%, because 2^⅓ is 1.26. That single fact explains why the last half-second is so expensive.
10 Cars, One Formula, and a 0.44-Second Blind Spot
Formulas are easy to publish and rarely audited. So here's Hale run against ten cars people actually look up, using curb weight plus a 180 lb driver and the manufacturer's flywheel rating, with typical magazine-tested figures alongside.
| Car | Weight / hp | Formula | Typically tested | ET error |
|---|---|---|---|---|
| Honda Civic Si | 3,130 / 200 | 14.57 @ 93.5 | 15.0 @ 94 | +0.43 |
| Mazda MX-5 Miata | 2,520 / 181 | 14.01 @ 97.3 | 14.5 @ 95 | +0.49 |
| Toyota GR86 | 2,980 / 228 | 13.72 @ 99.3 | 14.2 @ 99 | +0.48 |
| Ford F-150 5.0 | 5,100 / 400 | 13.61 @ 100.1 | 14.0 @ 99 | +0.39 |
| Subaru WRX | 3,570 / 271 | 13.76 @ 99.1 | 13.9 @ 100 | +0.14 |
| Ford Mustang GT 5.0 | 3,880 / 480 | 11.69 @ 116.6 | 12.4 @ 116 | +0.71 |
| Chevrolet Camaro SS | 3,865 / 455 | 11.89 @ 114.7 | 12.3 @ 116 | +0.41 |
| Dodge Charger Hellcat | 4,760 / 707 | 11.00 @ 123.9 | 11.6 @ 125 | +0.60 |
| Chevrolet Corvette C8 | 3,730 / 495 | 11.42 @ 119.4 | 11.3 @ 121 | −0.12 |
| Tesla Model 3 Performance | 4,230 / 450 | 12.29 @ 110.9 | 11.7 @ 115 | −0.59 |
Average trap-speed error across the eight combustion cars: 1.1 mph. Average ET error: 0.44 seconds, and positive almost every time. The formula isn't drifting randomly — it's consistently optimistic about the launch, because it was fitted to cars that got a decent one.
Two rows break the pattern in opposite directions. The C8 Corvette beats its predicted ET because a mid-engine layout parks the mass over the driven axle, so it hooks where a front-engine Mustang spins. And the Model 3 is the outlier that matters: its trap speed comes in 4.1 mph under prediction while its ET is 0.59 quicker. An electric motor makes full torque from zero rpm and then tapers, which is the mirror image of the combustion power curve these constants were fitted to. Feed an EV into any quarter mile calculator and treat the answer as a rough sketch.
Trap Speed Is the Half of the Timeslip That Can't Lie
Here's why that 1.1 mph versus 0.44 s split happens. Trap speed is recorded over the last 66 feet of the track, by which point the car has been accelerating for eleven-odd seconds. Spinning the tyres at the hit costs you time on the clock, but the car still ends up at whatever speed its power-to-weight ratio can push it to. Elapsed time counts the wheelspin. Trap speed has forgotten it ever happened.
That makes the equation reversible, and it's how racers estimate power without renting a dyno: HP = Weight × (MPH ÷ 234)³. Say a Mustang crosses at 12.84 seconds and 113.2 mph, weighing 3,880 lb with the driver aboard:
- 113.2 ÷ 234 = 0.4838
- 0.4838³ = 0.1132
- 0.1132 × 3,880 = 439 flywheel horsepower (about 373 at the wheels after a 15% RWD loss)
Now run it forward. At 439 hp and 3,880 lb the car shouldbe running 12.04. It ran 12.84. That 0.80-second gap never touched the engine — it was spent in the first 60 feet. Which is why swapping to a set of drag radials on that car is worth more than a tune, and why a horsepower and torque calculator will tell you what the engine is capable of but never what the tyres will let you use.
Read Your Own Timeslip in 30 Seconds
Enter your weight and trap speed in the calculator above, compare its predicted ET to what you actually ran, and the diagnosis falls out of the difference. This is the same logic a crew chief applies between rounds.
| What the slip says | What's actually happening | Where to spend money |
|---|---|---|
| Trap matches, ET is 0.3–1.0 s slow | The power is there. You're losing it at the hit. | Tyres, tyre pressure, launch rpm, rear suspension |
| Trap 3–5 mph low, ET slow to match | The engine isn't making the number on the brochure. | Tune, fuel quality, intake air temp, boost — not tyres |
| Trap high, ET quick, 60-ft under 1.7 s | Everything is working as designed. | Only power or weight will move it now |
| Trap matches, ET quick, 60-ft unusually low | The track prep is doing the work, not the car. | Nothing — expect to run slower on a cold, unprepped surface |
4 Things That Move Your ET More Than Horsepower Does
Because ET scales with the cube root, a 1% change in weight or power moves your time by only about one-third of a percent. Here's what that works out to in seconds, using the 3,880 lb / 480 hp Mustang as the reference car.
- Weight, and it's not linear across cars.Dropping 100 lb takes that Mustang from 11.69 to 11.59 — the old "100 pounds is a tenth" rule, which happens to be true at this power-to-weight. Put the same 100 lb on a diet plan for a 3,130 lb Civic Si and it's worth 0.16 s, because 100 lb is a bigger slice of a lighter car and that car's ET is longer to begin with.
- The first 60 feet.Trim 0.1 s off your 60-foot time and roughly 0.2 s comes off the quarter. A Mustang GT on all-seasons runs about 2.00 there; on drag radials, closer to 1.75. That quarter-second at the start is worth half a second at the stripe, and your trap speed won't change by 1 mph.
- Density altitude.A naturally aspirated engine loses roughly 3% of its power per 1,000 feet of density altitude. A hot afternoon in Denver can put DA above 8,000 ft, which turns 480 hp into about 384 — a 12.59 second run at 108.3 mph instead of 11.69 at 116.6. That's nearly a full second of weather. The National Weather Service publishes a density altitude calculator if you want your actual number for race day.
- Confusing wheel horsepower with flywheel horsepower.Type 400 whp into a predictor that expects crank power and it returns 12.42 s. But 400 at the wheels through a rear-drive car is about 470 at the crank, which predicts 11.77 — a 0.65-second error from one dropdown. The calculator above asks where your number was measured for exactly this reason.
Gearing is the one thing on this list the formula ignores completely. It assumes you can put the power down through whatever ratios you have, which is fine for a stock car and badly wrong for a swapped rear end or a transmission that drops you out of the powerband between shifts. Our gear ratio calculator shows where each ratio puts you, and the RPM calculatortells you what the engine will be turning as you cross the traps — useful for checking you aren't hitting the limiter 40 feet before the finish line. The NHRA and the history of drag racing cover the rulebook side if you're heading to a real track.
The 8.45-Second Rule That Turns Any ET into a 0–60
Line up published 0–60 and quarter-mile figures for street cars and something odd shows up: the difference between the two is nearly constant. A Corolla runs 8.2 to 60 and 16.4 in the quarter. A Golf GTI does 6.0 and 14.5. A Camry V6 does 5.8 and 14.3. A Mustang GT does 4.3 and 12.6. Subtract in every case and you get 8.2 to 8.5 seconds, across cars separated by 4 seconds of acceleration and 250 horsepower.
So 0–60 ≈ quarter-mile ET − 8.45is a better single-line estimate than any ratio, and it's what our calculator uses. The rule tightens up in the 12–16 second range and starts to compress below that: a 9.65-second car is traction-limited rather than power-limited off the line, so its gap shrinks to about 7.3. Above 17 seconds it drifts too, because a car that slow is still accelerating hard at the stripe. For anything you'd drive to work, subtract 8.45 and you'll be within about two-tenths.