What messes some people up is that most average speeds are calculated over time rather than distance. For example: If you do a 20-mile total out and back starting into the wind or uphill and average 10 mph on the way out and 20 mph on the way back, simple logic dictates that your "average" speed over 20 miles was 15 mph. But if you calculate based on time rather than distance you actually spent twice as long at 10 mph than at 20 mph bringing your average down to 13.33 mph. This can be demoralizing in that a relatively short hard climb, short section into a high headwind, or a bonk in the last few miles, can really suck down the average speed of an otherwise good ride. You'll really notice this in hills as the climbs may be the same distance as the decents but they take much longer so have a proportionally greater impact on your "average" speed over time.
BTW, my average on my road bike on a relatively flat course with minimal wind is in the 16 mph range for a 100-mile ride and about 18 on a 25-mile ride. I don't know if that is what the OP was asking, but there it is.
Last edited by GravelMN; 05-10-15 at 07:45 AM.