Get better with speed? Not terribly common, though sweet spots do exist for every car, and it just might happen that your sweet spot is at higher speed. With tall gearing, lugging/loafing around at 55 might not be as good as a sweet spot at say, 70.

Now our little highway car, with 2.0 liters and a six speed manual, mileage does go down as speed goes up, but, not by much (within the realm of our highway trips). Using GPS, our typical trip has a moving average of about 72mph, in reality this means ~90mph on the open road, and then brought down by congestion and/or surface streets. When we do this trip in this manner, we average ~31.5mpg. Same trip at slower speeds, moving average ~55mph, highway cruising ~70mph, results in mileage of ~33mpg. So while yes, the mileage is higher at the lower speed, we opt for the reduced drivetime.

I had another car years ago that was similar. I went on a road trip and for the first time in my life broke 30mpg, I thought this was amazing, and considered I was driving plenty fast, wondered how good I could do if I drove the speed limit instead. So the next weekend, I repeated the trip. At the speed limit. I got 1.5 mpg better. I was disappointed.

MV