Rollout or rollout allowance in North-American drag racing is the difference between actual acceleration time and measured acceleration time. For the published 0 to 60 mph acceleration time in North America, a rolling start is used, beginning 1 foot (0.3 m) after the initial standing start position. The method approximates the behaviour of dragstrip measurement equipment for 1/4 mile racing, which was historically limited to only recording after the vehicle had passed over a start line. The leads to a 0.2–0.3-second apparent difference, with larger wheel sizes giving a larger exaggeration in timing.
Historically acceleration measurement took place using dragstrip equipment placed over a measured distance, with a light gate at the start and end. These light gates measured the point in time at which a vehicle passed the measurement point, rather than the point at which movement (acceleration) first occurred leading to a slightly faster apparent time.
With the advent of on-board measurement via Global Positioning System timing, the North-American car magazines sought to standardise the precise measurements and to account for the historical difference in a way that was consistent between publications, and still comparable to existing published measurements for older vehicles made via traditional light gates.
The result was the standardisation of a "1 foot rollout [allowance]" (30 cm) derived by accurately measuring a complete acceleration run, then subtracting the time taken for the first 1 foot (30 cm) of vehicle movement, with only the derived timing being published.
'Rollout' is the distance travelled by a vehicle before the timing lights on a drag strip are triggered. … can affect the final run time by up to 0.3 of a second. … important to discount this first foot of movement from the final run time, to ensure that the run time captured by the GPS data logger is as close as possible to the official drag strip time.
length of the rollout depends on the diameter of the tire and where the driver chooses to position the car at the start. Although the rollout distance is typically only a foot or so of a quarter-mile, it can affect the elapsed time by as much as 0.3 second,
T= -0.26 seconds … 0.71g acceleration averaged over the first foot of travel … T= 0.00 sec … The car crosses 1-foot mark and official timing starts. The car is traveling at 5.9 mph, averaging 1.30 g of longitudinal acceleration. … T= 2.28 sec … The car crosses the official 60-mph mark
The time difference is usually due to … needless application of rollout. Rollout comes from the drag strip. When you start a quarter-mile run, the timing lights don't start until the front wheel is all the way over the starting line. … approximated by subtracting the first foot of an acceleration run. Now a common substitute is to cut the first 3 mph from a run, cutting as much as 0.3 seconds off the time. Some publications post a 0-60 mph with rollout and others do not.
"shallow staging" can therefore get almost a free foot of untimed acceleration before the clock officially starts, effectively achieving a rolling-start velocity of 3-5 mph and shaving the 0.3 second it typically takes to cover that distance off his elapsed time (ET) … rollout with 0-60 times is inappropriate in our view. … begun publishing a clearly marked "with rollout" 0-60 time alongside the primary no-rollout 0-60 time so readers can see the effects of this bogus practice.