I stumbled across a free piece of software the other day that I thought you guys might find interesting. It's called LapSim and is put out by Bosch. I haven't played with it much yet, but it's supposed to calculate lap times over a road course based on vehicle data. The accounts that I've read about people using it indicate it's fairly good, the calculated lap times are fairly close to what they've actually run. I'm not sure how useful it will actually be for autocross... but it should be fun to play with!