Too many variables to give a fixed answer. Depending on traffic, I drive the speed limit or 60-62, whichever is lower. If there is a lot of traffic I keep up with the flow so I don't become an obstacle, even if that means 70+ MPH. But once traffic thins out I back it down. Distance to be driven also plays into the calculation. I'm not retired (yet) so if I'm on a multi-day drive across country an extra 5-10 MPH can shave several hours off the total drive time which I'll gladly trade for the added fuel cost.