My ~5 mile commute is a gradual uphill ride (312 ft) so not very steep, but pretty constant. My average spd to work is typically about 11 mph. On the way home, that same gradual hill lets me easily cruise (in a velomobile) at just south of 40 mph. The speed limit is 30.
The cool thing about the velo is that the wind barely affects me at all. On nearly level ground, I can cruise at 23 mph into a pretty stiff headwind and not even notice it. The flipside is that tailwinds don't really help much either.
I've hit 65 mph in my velo on a short, steep hill near work. I think once I close the foot holes and get the race cap, I should be able to push 70 down that hill.