[mwilliamshs]

I've heard all my life that driving with as much manifold vacuum as possible is the best for mileage. I've tuned older cars and their instructions say to tune idle speed and mixture to highest indicated vacuum within an RPM range. It makes sense with a carb that more manifold vacuum (less carb throat airflow) would draw less fuel from the bowls.

BUT

Now I've started reading about pumping losses and throttle losses and I understand the benefit of EGR but it seems counter intuitive that high manifold vacuum (small throttle opening) would yield better mileage because the engine is working against the throttle.

SO

A high reading on my vacuum gauge means my engine isn't under a heavy load BUT it also means my engine is working burning gas to create that vacuum. So what's better, more manifold vacuum or less pumping loss?