I usually take 51 minutes 28 seconds to 52 minutes to commute 20 km using my singlespeed Dahon geared at 62 gear inches from door-to-door with or without the fairing. Pics of the setup here.

To test the theory that I should be faster using gears, I put back gears (SRAM 8 speeds) on the bike, lengthened the chain and tuned it up again. I even have more aerodynamic trekking bars that allow me to crouch into a more streamlined position. The idea behind gears is that I can go spin uphill and go faster downhill and faster on the flats. So I cycled with the same commitment I usually do today.

Today's time? 55 minutes 12 seconds. Longer time by 7%. My average speed went from 23.3 km/h to 21.7 km/h.

I don't understand. I really wanted this to work. I was faster without multiple gears and using just a flat bar with bar ends. I guess for city-riding it doesn't really matter. I will keep the gears on for a little while longer and compare the times from a few more rides.