Hmm! How do I put this
Hmm! How do I put this without offending. Thanks for supporting my contention about RPM efficiency brackets. But then you go on and use that to contend that because of the much reduced RPM in 6th gear of a GT-500 at 80 MPH, that it is getting better MPG. Apparently using low RPM as the only factor for good mileage.
The forces at play at 80 MPH would severely negate the efficiency of the engine at those low RPM's. The power it takes to maintain 55 MPH in a vehicle like a Mustang is around 55-60 HP. To maintain 80 MPH you need about 120+ HP. The caloric intake to produce 120 HP is significantly higher then 55-60 HP... It just doesn't pencil out in the real world.
In all practical terms/regimes, the 6th gear in the GT-500 is useless, unless your driving in Utah or Texas with their 80 to 85 MPH limits, and then...barely. And, you can't even reach top speed with it. But it helps to avoid the tax.