Okay, I think the information I was trying to determine has gotten buried beneath a physics discussion of drag, areodynamics and friction.
What I am more concerned about however is what RPM my 2.5 litre engine is happiest at. Each engine is unique. Run it too slow and it labours using more fuel for less output. Run it too quickly and it burns more fuel than it needs to for the speed at which you travel.
Think for a moment about riding your ten speed bicycle (more likely 18 speed now). If you pedal in too low a gear you pedal like mad and go slowly but get tired quickly. Pedal in too high a gear and you don't go as fast as you are capable of and still tire quickly. Find the optimum gear to pedal in and you go quickly with moderate effort.
Translate this to my 2003 Outback and under flat dry highway driving what RPM would my engine put out maximum horsepower while using minimum fuel? My Toyota Corolla used to purr at 3000 RPM. The specs for the Outback give maximum HP at 4200 RPM. Don't think I want to be running at 4200 RPM. Shell, Esso and Texaco might be happy if I did (for that matter so would Subaru) but I would be broke in no time and looking to drop a new egine in after about 150,000 km.
Any more thoughts folks? BTW, thanks for the feedback so far.
Best regards, Paul