Amen on the vacuum gauge. I bought a nifty "Mileage Minder" from ePay, it's hilariously calibrated in MPG as well as InHg, multi-color backlit and chrome cup. Fun, silly 50's gewgaw and useful to boot.
There's a complicated relationship of factors to get max. MPG at cruise; axle, cam and carb. My take on it is, you want cruise to be lowest RPM that has "the right amount" of torque to easily maintain cruise speed with a bit extra for mild hills, before you gotta mash it to climb real hills.
The cam-determined torque peak RPM is highest fuel efficiency (cylinder filling, firing, energy extraction, etc). Here's where I think the trickiness is: the engine is happy with the throttle open lugging at cruise, but the carburetor (...) will go rich at low vacuum, un-doing the low-speed benefit. So with carbed cars cruise is adjusted to be the lowest-rpm-that-maintains-vacuum-so-the-carb-stays-lean, I'm guessing but working at figuring out, that's a few hundred RPM over what might be optimum if the carb would stay lean.
I went EFI on one of my cars (amc 232ci six), but I'm not happy with mileage, though the BLM numbers are spot-on perfect. I'm gonna go EFI on my current toy ('63 Rambler American with the old Nash 195.6ci OHV motor) and will experiment more on that car. The first one was practice!
For the record, my little American gets 24MPG at 55-60 mph (1900-2000 rpm) and 22MPG at 60-65 mph (2000-2200 rpm). It's got a cam like a lawnmower, and a drag coefficient close to 1 (a brick!). I think the latter is the reason for the severe drop in mileage with speed.
