Smaller tires let the car break the beam sooner, taller ones later. A SuperGas car trying to get the car to break the beams as fast as possible would use a short tire to help. If you foot brake race, you can tune the cars reaction time by using different size front tires. The taller tires slow it down (reaction time), but the actual ET will be reduced by the amount the larger tires add to the reaction time. What you gain or lose from reaction time, shows up on the ET slip.