I ride 52/20 SS, my cyclometer read 13.5 avg with laptop on my difficult (hilly) route. on the easy route with no laptop I usually average a one to 1.2 mph faster. If I'm racing sunset to get home, I see something like 15.8 as my average for a ride home (above averages are for the round trip)

I'd say my modal speed(the speed I see on the computer the most) is 17-20...the hills and stoplights eat my average.

I am however, hauling a heavy bike, rider and backpack