Here's a BSFC chart for my engine:
I generally drive at 50mph when the speed limit is 50 or above, which puts me right at 2000RPM, where the engine can most efficiently convert fuel into motive energy. From the chart, you can see that above ~75% load, efficiency begins to drop off, so it's best to accelerate at high but not full load.
I have a $3 eBay ELM327 bluetooth dongle paired with my smartphone I used to keep an eye on load until I got a feel for it.