On a dare, I tried driving no faster than 65 mph for an entire tank of gas. In my car, that's about 13 gallons, and at my usual speed of 73 mph, it lasts for about 330 miles (25 miles per gallon). I'd get on the highway, set the cruise control to 65, sit back, and... get really bored.
It turns out that a 8 mph difference is way noticeable. Whereas I was used to the trees speeding by me (or vice versa, as it may be), and keeping up with traffic, suddenly everyone was passing me and I was going slow enough to see the grass grow in the emergency lane. I had to drive in the slow lane, and even there people would creep up beside me and idiotically expect me to get out of their way. Honestly, I did speed up every now and then, to pass someone going even slower than I in a timely manner, or to avoid people who don't know how to merge from the on ramp. But mostly, I just watched people and tortoises zoom by me.
I also watched the odometer. I still had plenty of gas when it passed 330, and long after that. My low gas buzzer went off at 420 miles. Four hundred twenty miles. I went 90 miles farther, simply by going 8 mph slower. That's over 32 mpg, or a difference of 7 mpg. Or talking about money, I got an extra $11 out of that $40 tank.
And I didn't really even lose appreciable time. I had the opportunity to check this out. About 15 miles from lab, I noticed my lab-mate pass me on the highway. He drives 75 or 80, and I arrived soon enough to get the parking spot next to him. The way our garage fills up between 9 and 10 am, that means I arrived less than 5 minutes later.
I'm willing to give up 5 minutes in order to save $11. I'm cheap like that. And I've actually gotten used to the speedlessness of 65 mph. It gives me more time to read.