20MPH average is a sort of benchmark for racers doing solo training, a "minimum price of entry" to ride in "A group" training rides....... at least it was when I raced, so things may have changed.
The basic point is that a 20MPH average is a solid pace.
If you are a strong rider and are commuting on a fairly efficient bike, I'd say it is definitely achievable. If your average is in the 14-16 range now, I feel you would need to employ a "training" mindset and technique to your commute to get your pace up. Alternate sustained "pace" days with "intervals" days 1-2 days per week, and so forth. The road racing and training forums would be a god source for that kind of info.
As you've acknowledged, alot depends on the "motor". I (and others I know) could maintain a 21+ MPH average pace for a 40 mile, relatively steady (not many stops) route on mountain bikes with slicks. We were racers, though, and our solo training pace was 22-24 MPH (23-28 average in groups) , and we rode 13-15 hours per week on a very strict training routine.
Is it possible? Of course. Is it possible on a loaded commuter rig? Of course, given the nature of your route. The biggest factor is the engine. Time to start thinking of your commute as "training" if you want it to happen.
Good luck!!!!
Last edited by canyoneagle; 03-02-11 at 11:56 AM.