Because my wife gets to drive her Hybrid solo in the car pool lane here in California, she has to drive at 70-80 to keep from causing a bottleneck. Her current average is about 39.

If you have the luxury of being able to hypermill, then you can get the numbers up very easily, but in a congested freeway where the lanes are totally clogged or driving at hyperspeed, then your numbers are going to be in the high 30s low 40s.