[Jack21]

I use a vacuum advance distributor, and have on every street machine I've ever built.

The additional advance at high vacuum, low load cruise increases fuel economy.

However, when you dial centrifugal advance in sooner, you have to reduce the amount of vacuum advance so the combined centrifugal, and vacuum advance do not over advance the timing.

OEM distributors didn't have all the centrifugal advance in until perhaps 4000 RPM. This is way above cruise RPM, typically passing. When accelerating, the vacuum, and vacuum advance dropped out so the engine did not have full centrifugal, and vacuum advance at the same time.

At cruise (2300 RPM, high vacuum), the engine had full vacuum advance, but not much centrifugal advance. Again, not both at once.

In a warm street engine, when dialing in a distributor that will have all its centrifugal advance in at cruise RPM, the engine needs some, but less vacuum advance to optimize cruise RPM fuel economy.

Think if it this way. Centrifugal advance optimizes power at wide open throttle (low vacuum). Vacuum advance optimizes fuel economy at cruise RPM (high vacuum).