I drive whatever speed traffic is going, or a little faster if there's enough space to do so without threading through lanes like a dick.
When there's no traffic around, I drive as fast as I think I can get away with safely. On a nice day, that usually means matching my speedometer with the speed limit signs exactly ... although my car was made before Canada went metric, so you do the math.
My fastest was on Ontario's toll highway, the 407, where traffic is very thin and very fast on weekends. I used to go 180-200km/h on there regularily when I had a turbo, and I got it up to 230 once to pass, with the left blinker on Autobahn style. I don't do that anymore though -- the car I drive now couldn't go that fast without a booster rocket.