[elhigh]

You can't just say the engine is going to deliver its best FE at such-and-such RPM. The entire package has to be taken into consideration - how fast the car is going at those revs, what the torque curve is like, etc.

If you've got a ScanGauge, you can do some quick-and-dirty testing and find out for yourself exactly what is the very best possible speed for steady-state mpg, then come back to your friend and tell him that, in your car, Xmph at eXrpm delivers the best fuel efficiency. Then overwhelm him with data to make him go away.

When I get my vacuum gauge installed - I'm looking at it right now! - I'll be able to tell whether my seat o' the pants estimate of 43mph is indeed the perfect happy-happy cruisiing speed for my Toy.