gear inches: http://en.wikipedia.org/wiki/Gear_inches

many people say that around 70 gear inches is a good average to shoot for. you can calculate gear inches here: http://software.bareknucklebrigade.c...it.applet.html

I used to ride 40x16 on my singlespeed as well, and I agree, it was easy to spin out, at only 65.9 gear inches. If your setup would allow for a 44 tooth chainring, that'd boost you to 72.5, which also will increase your max speed some, prior to spin out.