As someone who has spent a LOT of time boosting engines I see boost with the cruise control on all the time.
Hills gentlemen, hills. Some cars like the SRT-4 see boost off idle, ridiculous. You can size a turbo so large that you almost never see boost at cruise but that does take some of the fun out the package.
I agree with the poster who already pointed out that the mpg gains come from the ability to run a smaller motor at cruise and a "larger one" as needed under boost.
An example I would use would be the 2.2 engine at 0 boost would be using as much air as a 135 CID engine. At 14 psi indicated boost (twice atmosphere) using as much air as a 270 CID engine and so on.
Simplistic, but you get the general idea.
a turbo 4 banger might pull boost at cruise but a BB will not. I had many many highway and cruise miles on my old S/C'd 440. It never pushed any boost at speed. With vacuum readings of 20+ how can it? It would go into boost until vacuum was near 0.
