Drag racing is run over two standard distances. The quarter mile is 1,320 feet, which is 402.336 metres. The eighth mile is half of that: 660 feet, or 201.168 metres. Many strips are built for one or the other, and the timing equipment on a quarter-mile strip also records the eighth, which is why a time slip often shows both. This post compares the two, explains why the times do not simply double, and gives a rough way to convert one into the other.
Why twice the distance is not twice the time
A car starting from rest and accelerating at a constant rate covers a distance d in a time t where d = ½ × a × t². Rearranged, t is proportional to the square root of d. Double the distance and the time grows by the square root of 2, which is 1.414. In that idealised case an 8.00 s eighth mile would become an 11.31 s quarter mile.
Real cars do not hold a constant acceleration, and the difference runs one way. The launch is the strongest part of a run, and as speed builds, air resistance grows and each gear change interrupts the push, so acceleration falls away. The second half of the track is covered at a higher speed but with less urge, and the quarter-mile time ends up further above the eighth than the square-root figure suggests. How much further depends on the car and how its power is delivered, which is why the number below is only a guide.
The rule of thumb
A common rule of thumb for converting an eighth-mile elapsed time (ET) to a quarter-mile ET is to multiply by about 1.55 to 1.57. It is an approximation. It tends to work reasonably for ordinary cars on a good surface, and it can be out by a few hundredths of a second or more for a car with an unusual power curve, a lot of wheelspin or a gearing problem. The table applies it to four example eighth-mile times. The times are illustrative, not from any particular car.
| Eighth-mile ET | × 1.55 | × 1.57 |
|---|---|---|
| 6.00 s | 9.30 s | 9.42 s |
| 7.00 s | 10.85 s | 10.99 s |
| 8.00 s | 12.40 s | 12.56 s |
| 9.00 s | 13.95 s | 14.13 s |
The example slip in the time slip post sits right at the top of the range: 7.12 s at the eighth and 11.20 s at the quarter gives a ratio of 11.20 ÷ 7.12 = 1.573. The second half of that run took 4.08 s to cover the same 660 feet that the first half needed 7.12 s for.
The ratio of the two times is a quick check on a run. A quarter-mile ET far above 1.57 times the eighth suggests the car faded late; one well below 1.55 suggests the early part of the run was the weak one.
What the two distances reward
The first 60 feet make up 9.1% of an eighth mile but only 4.5% of a quarter. A bad launch is therefore a larger share of the shorter run, and it is easier to see in the result. The 60-foot time post covers why that early stretch carries so much weight. An eighth mile is mostly a test of launch and the first two gears. The quarter mile adds the part where gearing, top-end power and the car's drag all matter more, and where a car that has been shifted too early or too late has more time to lose. That is also where trap speed is more telling: it reflects how hard the car is still pulling when it crosses the line.
Distance also has a practical side. A shorter track needs less room and leaves more run-off beyond the finish line. A car that crosses the line at 100 mph covers about 147 feet every second, so the stretch after the timing beam is not generous on a short strip.
Where 1,000 feet comes in
The NHRA's Top Fuel and Funny Car classes moved from a quarter mile to 1,000 feet in 2008, following a fatal accident that year. At 1,000 feet, 304.8 metres, the run is 75.8% of the quarter mile. Those cars are fast enough that the shorter distance leaves more room to stop. Other classes continued to race the quarter mile. If you see a 1,000-foot line on a slip, it is a timing beam for those classes and usually sits between the eighth and the quarter.
The rule of thumb above does not carry over to 1,000 feet. It was built for converting between the other two distances, and a car that accelerates as hard as a Top Fuel dragster does not behave like the examples in the table.
Using it when you tune
If you change one thing and want to know whether it helped, compare the same distance each time. The eighth-mile time isolates the launch and early gears, so a change to launch RPM or tyre pressure shows up there first. The quarter-mile time then tells you whether the change cost anything at the other end. In Pink Slip Kings, whose listing describes race time slips, telemetry and a Test & Tune mode, the same habit applies: change a single setting, run again and read the slip lines before and after it.
Pink Slip Kings: Drag Racing is a 3D drag racing game with manual launch and shift controls. You choose your launch RPM and time your shifts, and the listing includes a dyno, telemetry and race time slips, plus Test & Tune and tuner sandbox modes. It plays offline, has no ads and no in-app purchases, and its in-game cash and cars cannot be bought or exchanged for real money.