I find about 60ish is the optimum speed too. Usually, the traffic is moving somewhere between 50 and 70 anyway and I don't see the fun in blatting down the outside lane at all to be honest. I also know it makes very little difference to the amount of time it takes to get to work. A colleague of mine over takes every day not long after I pull onto the motorway, he goes past doing 80 and I am usually the car behind him at the factory gates! The difference in fuel economy between doing 60 and 80mph is usually large due to drag, but it will depend on the gearing of the car etc there are lots of factors. Stick to a speed, accelerate and brake as little as possible, keep up momentum.
Tyre pressures can be another quite large factor.