There seems to be a lot of differing opinions on what average means. That one time when you went XXmph for XXminutes isn't indicative of anything. If you have Strava or any other tracking site, the numbers are right there. If you use a basic cycle computer, guess what, the numbers are there, too. You should have an odometer and a total time elapsed. Divide the miles by the hours, and there you have it. THE AVERAGE. That said, it's not a metric I regard as important. Miss a lot of stop lights, speed is going to be down. Ride a lot of hills in few miles, speed is going to be down. I look at the total mileage, not how long it took to get there.