All my local driving is on 2-lane roads, with 55 mph speed limits. In a concession to my age and the number of deer and bears locally, I generally drive them at maybe 50 to 53 mph, unless cars are behind me. Much of the driving is up and down hills, many in the neighborhood of 8% or 9% grade. A lot of curves which require slower speeds then accelerating again.
On expressways, I will drive 62 to 65 instead of 70 mph if traffic is not too heavy and my mileage does not seem to suffer much. If I drive at 70 mph, I notice a big drop in mileage.
I am late-70s and retired, so I have the luxury of allowing an additional 5 minutes to arrive.