Why measure to a millisecond?

The derby timer uses a microprocessor that can process millions of instructions per second.  Therefore making the derby timer measure in increments of less than a millisecond (0.001 second) is easily doable given the timer electronics.  So, why not go for 0.0001 of a second?

Consider a track that is 28 feet long, with a starting line that is 4 feet above the finish line.  A car that finishes the race in 2.8 seconds has an average speed of 10 feet/second, but is going about 16 feet/second at the end of the race.  In one millisecond, the car will cover 5 millimeters (1/4 inch).

The small laser beams used at the finish line are about 1 mm in diameter.  In order to measure to 0.0001 of a second accurately, the lasers would need to be aligned within 0.5 mm.  This could be done on the workshop bench, but would prove a little tricky (and unnecessary) in a gymnasium full of cubs scouts.

Of the 72 heat times recorded at my son’s pinewood derby, 30 head-to-head times were within 0.01 second on the same lane.  Only 2 of the 72 times were within 0.001 second on the same lane.  Measuring to 0.001 second makes the likelihood of a tie very remote, particularly in a race-off situation with only a few cars competing.

Conclusion: One millisecond is an ideal level of accuracy that is relatively easy to measure and does a good job of discriminating between close finishes.

Another thing to consider is that displaying 0.001 second requires one less character on the LCD display than 0.0001 second.  Display space is expensive.  You will want the largest display that fits your budget, so use the space wisely.