I kept some accurate stats over the last couple of thouseand commuting miles, so here's the bare bones truth of it....

Overall average speed............................27.7km/hr (17.3 mi/hr)
Fastest single commute, no wind..............33.2km/hr (20.75mi/hr)
Slowest single commute into headwind.......17km/hr (10.6mi/hr)
Highest speed........................................79.9km/hr (49.9mi/hr)

My fastest commute was on a Monday morning, the slowest was an evening late in the week, into a howling southerly (I live due south of work)