Scouse, it's a moot point. Depends entirely on too many factors. The average speed I quote you would be for riding where *I* live, i.e., sea level, flat, hot, humid, usually windy. It would have no bearing at all on a rider in Denver, i.e., 5000', hilly to mountainous, mild to bitter cold, yada, yada, yada.

Do a search on google for "your city cycling club", whatever your city is, (or county or region). It will most likely net you a few listings of clubs that have "rated" rides in your area. They usually post right on the club's site all the information cornucopia72 was talking about and more. Several club rides around here even state the actual ride speeds.

For instance, check our local Suncoast Cycling Club's C+ group ride. It states an 18mph max speed, and they make everyone stick to that. The ride is both a learning and a social ride, has a no drop policy, and the ride leaders make everyone adhere to the 18 mph limit even on downgrades. The idea is to allow the new guy to be certain he can keep up first of all, and secondly to give him a chance to get used to group riding before he finds himself shoulder to shoulder at 25 or 30 mph.

That's the kind of local club you want to find! They are usually the bigger clubs, so you get to meet more cyclists as well as work your way up through the rides so that by the time you're showing up for the fastest rides they already know you belong there.