Unless the conditions are rooted (monsoon, fog, etc) I like to think of most speed limits as a LOWER limit, not to go under if it's a good situation (i.e. not infront of a school at 3:25pm). The upper limit is entirely up to each individual driver, and how lucky he/she is feeling about speed cameras/radars . .
Most cars return their best economy between 70->90km/h, depending on gearing and aerodynamics.