Vacuum is kind of a rough measure of "how hard" the engine is working. There is an argument that more vacuum helps the PCV system to work, and so reduces chances of gumming the oil. I don't know if that makes sense personally, seeing as the PCV joins the intake before the throttle. Another aspect is that a "harder working" engine sees more stress and that may not be a good thing. More stress may or may not translate into faster wear, supposedly depending on whether the oil films can still maintain clearance.
As for fuel economy, the engine has a performance curve for that like it does for horsepower. I haven't seen it, but would guess there is a good chance that a lower gear might get better mileage than lugging the engine.
As for fuel economy, the engine has a performance curve for that like it does for horsepower. I haven't seen it, but would guess there is a good chance that a lower gear might get better mileage than lugging the engine.
