Originally Posted by fireball_jones
Lowest possible speed in your highest possible gear.
This is absolutely incorrect!! Use the bicycle analogy, pedal in top gear at the slowest speed you can go and you're really cranking on the pedals to keep going. Cranking = energy expenditure which in a car = fuel consumption.
All engines have an RPM range that gives it its maximum efficiency. For a bicycle that might be 90 rpm (the human is the engine) for a lawnmower it might a 3,000, for a jet engine 80,000. For a Subaru engine I seriously doubt it is the "lowest possible speed in your highest possible gear" which would probably be about 1,500 rpm on a flat road. My guess is around 3,000 is more like it.
Now you have to factor in aerodynamics, which means keep it under about 50. Ignore rolling resistance at varying speeds although my old dynamics teacher would disagree.
So my somewhat-educated guess is what someone said before, 40-45 mph in top gear.