Along with the above, it also depends on what the gearing is on your car.

My 4be is happiest sitting around 55-65mph where it will return around 40mpg. If I drive it around 70-75mph, it will average around 33mpg. 80mph+ and you are into the mid-20s.

However, my cabriolet on its twin-choke Weber will return about 35-40mpg around 70-75mph (as long as the second choke is not on when it returns about 15mpg regardless of speed and the roof is not down) but will return around 20mpg @ 55mph.

The difference is that the 4be is doing 3k rpm @ 70mph, but the cab is doing 2,200rpm @ 70mph due to the gearing of the diff. If I sat the cabby @ 55mph, I would have to either change down or use the throttle more every time I came to an incline, which in turn uses more fuel compared to sitting @ a higher speed where you do not need to drop a gear.

If you take a Sierra DOHC and do side-by-side comparisons (literally 2 cars next to each other) with a 3.92 diff and a 3.62 diff, the 3.62 diff will be doing 5mpg better @ 70mph, but the 3.92 will be better around 55-60mph.

Something my father told me years ago as well was that if you can drive around 500rpm below max torque, you should be getting the best fuel economy. Seems to work for me.