The way I look at it, Low pressure, the tire bulges out front and back, at rest car would be a tad back farther when touching the beam, like when the old racers staggered the wheels to pick up et. Since the car rolls farther to get out of the beam, reaction time will be slower, the other way with the high pressure, tight, not much bulge, relation time is quicker. I think I read somewhere that measuring from spindle to ground = radius, 3.14 X radius squared gives the true diameter of the tire. Personally I think it's a waste of time, I varied my reaction times by launch rpm. Foot braking, Each 100 rpm was equal to one hundredth. 1900-2100 - .01- .0 -red