My average speed is alwasy 2-3mph below my cruising speed. I often cruise at 20mph, but with stops, turns, and other reasons to slow down, speed up, my average speed tends to be about 17.

Interestingly, in my car, my average speed is 34, so my bike is exactly twice as slow as my car. A 30 minute car drive will take 60 minutes on my bike (and it does). The bike does have the advantage of not having to deal with traffic and parking though.