The speed of the car is only a piece of the puzzle, its the speed of the engine and where you are in the power curve that determines fuel efficiency. the m104 engine is most efficient (power to fuel used ratio) between 2.5 and 3.75k rpms.

The 3.2 L (3199 cc) version produced 220 hp (161 kW) at 5500 rpm and 310 N·m (229 ft·lbf) at 3750 rpm

According to "How stuff works" the best place to shift gears for fuel economy is the max torque rpm. That means that in the m104 you're nearly at 4k before you shift for max economy. On my 606 however I should be shifting at 2200 rpm. Such is the difference between gas and diesel I guess.

My hypermiling has always been just upshift as fast as you can and don't push down on the accelerator if you can help it. I'm wondering if I'm missing out on free power by upshifting too early and hanging out in lug territory too long.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg