An engine that produces high vaccume at idle is makeing very good use of the air and fuel going through it, the problem with high vaccume is during cruise, the high vaccume is fighting the pistons downward movement, just like engine brakeing. The reason a vaccume gauge helps MPG is because it tells you when you are not putting an un-nessacary load on the engine. The trick to getting high MPG with a big engine is to build it to make lots of TQ at a very low RPM and lug the heck out of it with gearing to get the vaccume down at cruise speed, that way it is not wasting energy pulling a vaccume in the intake