I think really depends ... I have only one speed (44x16 w/23-622) and a short commute of roughly 9km and flat with one climb at the beginning.

I usually average between 28 and 32km/h depending on weather ... so about 30km/h or 18.6mph ... according to Strava.

That's what speed the bike feels comfortable at (somewhere around 90rpm cadence).

I haven't reset the "trip" average speed since I bought the bike and with the walking around and everything ... over the first 900km it's at 26km/h average (16 mph).