I'm just a budding CSE major, but I see 4 sets of IR sensors. The first 2 IR receivers are placed at the start, and are spaced from each other a few inches or so. These are used to line up cars evenly and accurately, and to make sure every race starts at the same location. The other 2 are placed at the finish line, 1/4 mile away from the start sensors, and spaced from each other similarly to the start line. All sensors are placed near the ground, and only "see" a car's tires.
When a car starts the race, the first beam is broken but the second is still in tact. You start timing as soon as the second beam is broken, and you stop timing as soon as one of the beams at the finish line is broken. I put 2 IR sensors down there to solve the "the cars move too fast" problem.
All of these sensors would hook into some computer system and as long as they all act as switches, I don't think it would be too difficult to program for them. If the sensors are quick enough, you could potentially use the 2 finishing sensors to calculate ending speed, but I am unsure of the accuracy.
And if your using WiFi N, you could always use some sort of cantenna to point your signal down to the start to help with signal strength.
My $0.02.
-Cool-