Using a vacuum guage and gear selection towing for best MPG?

I think that it's less of an issue with fuel injection than with a carburetor. The carburetor had a "Power Valve" that opened at set vacuum point, FI controls the mixture with the O2 sensor in the exhaust. So while FI it may still enrich the mixture, it does it on the curve. Not at one set point. So less RPM, less fuel, IMHO.