Larry,
I will quickly agree that the engine is performing best when vacuum is high.
I'm not a disagreeable person. I just like discussing this stuff since it educates my brain.
However, I don't think we actually drive that way. What I mean is, if we are on the freeway, we stay at speed with traffic. Or at least we try to. If we are out for a drive on a country road, we adjust speed according to the curves and road conditions. We usually stay right up near the speed limit if the truck handles well enough to do so.
Here is how I think. My 65 C10 with factory 3 speed would turn 2031 RPMs at 47 MPH. The RPMs would be 2809 at 65 MPH. I think we all would agree the truck would get better MPG at 47 MPH than at 65 MPH. Wind resistance would play a huge factor. I don't know what the vacuum gauge would show at either speed. The other side of the coin is to say, If the manifold vacuum was higher at 2809 RPMS, I doubt you'd agree that speeding up to 65 would save gas.
My real life example is this. My 1965 C10 can now travel at 65 MPH at the same RPMS that it had at 47 MPH. That should improve MPG since I still have a Rochester B. I might do a test. I might drive it and just use 4 gears and no overdrive. That would be just like the 3 speed with 1:1 in 4th gear. Then see if my MPG drops or improves.
Folks with different carbs may see different MPG changes because of the variables in how and when certain carb fuel circuits engage based on vacuum.
