According to this post from the other thread, the highest vacuum is not the only thing you take into effect when figuring out the best mileage:
"Way back at the top of the post, someone made the statement about the highest vacuum producing the best mileage- that may be how you are advised to drive to get the best mileage, but you are loosing volumetric efficiency on the engine- working too hard just to suck air-
The best mileage comes from having your throttle open very far, usually in a high gear at low RPM's- of course, our vehicles are typically so powerful, that, when you open the throttle, even at 1500 RPM's in high gear, they begin to accelerat- think of like a 1 liter Opel or something similar, running along at 50-60 mph, in overdrive, and you will get the most efficiency holding the throttle open-
Wind resistance, which is the primary thing you are burning fuel to overcome at highway speeds, is logarithmic- if speed doubles, resitance quadruples- things like rolling resistance and the drag of the axles and gearbox pretty much remain constant at a wide range of speeds- This is why small aero mods can make large percent differences in mpg's"
Here is an example from someone in that thread with a C5 Vette (an aerodynamically efficient vehicle).
"My Corvette definitely does not get better gas mileage at 90mph! You can clearly see it on the instant MPG readout. Mine seems to get it's best mileage somewhere around 50-55mph. At that point the instant MPG will read around 34-35MPG if you're on perfectly flat road."
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