Mostly when I (infrequently) reset the average speed it reads around 26.x km/H, about 16 m/H. This is for my commuting hybrid which is heavy-ish and is decked out with fenders/carrier/toolkit/pump/lights/hub dynamo etc with 700/37 el cheapo tires. The commute is 35 km (22 miles) each way and the town I live in is 200 metres above the town I work in, so going to work is always faster than coming home, although there's no hills of any decent size, only a few short rises. It's usually blowing about a minimum of 15- 20 km/H in one direction or another, so generally one ride will be harder for this reason too, although the prevailing westerlie is a cross wind.
When summer really kicks in ( I'm Southern Hemisphere) I'll start taking my old 7 speed road bike I bought cheap second hand, on the night shifts, as I won't need lights then. I'd like to think I could average 30 km/H there and back, maybe more if conditions are favourable.
BTW my fastest time down was 50 minutes (42 km/H) and my slowest up 2 hrs 35 minutes (13.5 km/H). Both were in very strong winds. 🙂