The range of RPM is a general
The range of RPM is a general comment, but widely applicable. There are variations to the norm, but not much, this is due to a number of factors and would have to be considered individually. Higher compression ratios raise the volumetric efficiency at higher rpms. Higher altitudes lower volumetric efficiency. For example, my BMW 328is(10.2 CR) gets as good as, or slightly better mileage at 65, then 55 @ near sea level. It also exceeds the EPA Hwy figure by 5-6 MPG. This because it's volumetric sweet spot(efficiency @ near sea level), is such that it over comes the negatives of increased coefficients of drag, mechanical frictions, External frictions, internal frictions, and rolling frictions, But a 65MPH it is only turning over 2,300. Beyond 65 the combined cumulative forces severely reduce efficiency.
Overdrive sixth gear(s) are used by MFG's to get good EPA figures and avoid the Gas
Guzzler tax. They are generally useless in normal use, unless your going down a long steep hill at speed. What the MFG's don't tell you, is, the sixth gear(and most 5th gears) are generally very weak, comparatively, and misuse or abuse will cause premature failure.
In sixth gear, the GT 500 won't pull max RPM or top speed, that is attained in 5th gear with the stock final drive ratio. The 2000 Cobra 'R' would not pull max RPM or top speed in 6th gear with the 'as delivered' 3.55 final drive ratio, while my SVO( same 'OD' ratio 'aftermarket' 6-speed T-56/3.45 FDR) would, due to its higher rpm torque, due to turbo charging, and its final, overall, drive ratio. I have yet to see few cars that would, even given enough room on a calm day at reasonable temperatures. Land speed cars, have to be geared for their target speed plus a little more, based on their RPM @ max HP, factored barometrically for air pressure/density, and temperature, and length of speed course. Same for drag cars.
RPM at a given speed, can vary widely in an automatic without a locking converter. The final high gear, 'low' ratio, whether a 2-speed or 3-speed auto, was always 1-1, and is not factor as to engine efficiency at a given RPM. My last 2-speed glide was in my Altered roadster.
Gearheads commonly confuse the gear/ratio issue. Low gear is a high ratio, High gear is a low ratio