My truck ('92 Bronco 302 4-speed auto OD) seems to like 45mph best, that puts it right at 1500rpm. Torque peak is 270 @ 2400. My understanding is that efficiency also depends a lot on the amount of vacuum you pull; the lowest RPM at which you pull peak vacuum is the best engine speed for mileage. Hence why some of the early gas mileage meters went off of engine vacuum.

Best proof I have of this is my Dad's Passat he used to have. It was EPA rated ~25mpg freeway, but at exactly 42mph (I believe it was about 1800rpm? Peak torque was 206@3200), it got 48mpg according to the onboard mileage computer thingy. Optimistic maybe, but even if it were reading 20% high there, it still says to me that engine vacuum plays a huge role.