[hotrod 80]

I tried to write somrthing up , but this is from Summits web site . It explains it much better than i could .

Adjusting Rollout With Tire Size
You can modify your rollout by changing the size of your front tires. The
height and width of your tires, measured at the height the staging light
beams are from ground level, determines the amount of rollout. This
illustration shows where the lights are set at a typical track sanctioned
by the International Hot Rod Association (IHRA). The IHRA sets its light
beams approximately 1 5/8 inches off the ground. The larger your front
tires are at that height, the more rollout you will have.

Here's an example: Let's say your front tires measure 12 inches across at
the light beam height and you are using a shallow staging technique. When
you launch, you have a “rolling start" of 12 inches before you pass the
stage beam and trigger the ET timer. If you replace those tires with ones
that measure only eight inches across, you have effectively decreased your
rollout factor by four inches (12–8=4) and also increased your chances of
cutting a better light. Remember, less rollout means better reaction
times.

You can also alter tire size by playing with air pressure. More air
pressure increases tire height, but reduces width and the size of the
contact patch. That will reduce your rollout. Conversely, reducing air
pressure reduces tire height and increases tire width and contact patch
area. As you probably guessed, that will increase your rollout. Air
pressure is an easy and inexpensive way to see how tire size affects your
staging technique and reaction times; the slide show has photos showing
the effects of tire pressure on rollout.

Like anything else, the best way to apply these staging and rollout
techniques is to practice, practice, practice. Once you find a staging
routine that works for you, stay with it. When you stage the same way for
every run, the more consistent you will be-and consistency wins in bracket
racing. Happy rollout hunting!