I know my cruising speed is under 20. I don't have a speedo, but I've verified my speed a few ways. I have Bell Wear Alert tires that have a green stripe that turns to red when they wear down. So I've counted the tire revolutions for 30 secs and computed that way. You can also count pedal turns the same way without staring at your front tire.
Since I have friction drive with a 1 1/4 inch roller, my rpms must be about 5400 at 20 mph. I was thinking, since I don't have a tach, I can use my digital camera to record the sound of my motor running, and check out the waveform on GoldWave to see the rpms. (But I'm pretty sure it's gonna' come out around 5000 anyway).