+1 to bscar’s comment.
Yes, we all want to save money, but if driving at…55 mph…or 60 mph…or even 65 mph means that you are moving significantly slower than the rest of the vehicles on the road, you are putting yourself in imminent danger of being rear-ended by a vehicle moving much faster than yours. That would pretty much override any cost advantage that you might have gained by driving at 55 mph.
That being said, I have spent some time over the past 23 months or so, monitoring the mpg readout on my 2011 car. By comparing the average mpg readout to actual hand-calculated mpg, I have found the onboard readout to be extremely accurate, with an “optimistic” reading of less than .25 mpg.
That being said, when I switch the readout to “instant mpg”, instead of average mpg, I find that the road itself has more to do with the car’s economy than my right foot does. If I am driving at a steady speed of 55 mph on a level road, I will usually get an instant readout of ~30-31 mpg. However, if there is an uphill grade that is encountered on that same road, the instant readout might drop to as low as 17 mpg.
Similarly, I can drive at a steady speed of 65 mph on a level road and get an instant readout of 28 mpg, but once there is an uphill grade, that suddenly goes down to ~15-16 mpg.
So, since most roads (at least in my neck of the woods) do vary in grade, and since I do want to at least keep up with the prevailing speed of other vehicles, in the interest of safety, I tend to drive at, or slightly above, the prevailing speed of the traffic on the road.
In other words, the grade of the road is going to have more of an effect on your mpg than anything else, so I would suggest that you keep safety paramount, rather than trying to wring that last fraction of a mpg out of that tank of gas.