I've seen some lap timing printouts that show a drivers "consistancy" for their run. I wish it was calculated on the sheets at my local track. I'm pretty certain the formula used is known as a standard deviation. If you want to do it manually...

1. Each laptime - avg. laptime then squared
2. Average of step 1 results
3. Square root of step 2

If you don't want to do the math for the entire run (every lap), you can use the sample standard deviation which is nearly the same formula except step two would not use the total number of data points (laps), but instead use one less. That formula is suitable for any number of data points you want to use less than the total population.

Obviously this is considerably more work but if one wants to compare to what is calculated on those timing systems that do it.. That's how it's done.... I think.