I ran into a similar issue with a 2003 Honda Accord (v6 manual trans, one of my favorite cars ever). I was spending almost 4 hours a day in the car for a three

I ran into a similar issue with a 2003 Honda Accord (v6 manual trans, one of my favorite cars ever). I was spending almost 4 hours a day in the car for a three month job and went through all sorts of experiments to find peak efficiency. If I was on a level section of road, I could get better mileage by driving slower, e.g. 55 vs 75. But when I started to negotiate hills and have to accelerate to maintain speed, the mileage dropped way down because the engine wasn't efficient at generating power at those lower RPMs. And this is where you start to see these unexpected patterns in mileage with gas/diesel engines...they have a very lumpy efficiency curve, and it's regularly more important to stay in a specific RPM range than a specific speed if you want to maximize mileage. Once you start going up to 80-90-100mph physics takes over, of course, but there is a lot of variability in between. At the time I was just thinking fuel prices and it got to the point where I was calculating how much I was saving by driving slower and it was cents per hour. I ultimately landed on 70mph as the sweet spot, and I would regularly get 32mpg.