I'm having trouble increasing the FE of my Protege, and I think I may have found part of the reason why. Driving home just now, I was on a street doing 40mph with the cruise control set. Level ground, and my FE was 33.4mpg. I used the cruise control to accelerate a bit. Monitoring my SGII, I saw the FE drop from 33.4 to 17.4 during the acceleration. Roughly half.
The math: Half the FE means twice the fuel, right? Assuming a steady-state fuel mixture of 14.7:1, doubling the fuel would richen the mixture to approximately 7.35:1. That's incredibly rich. Enrichment shouldn't drop the mixture much below 12:1. Forced-induction setups can sometimes require richer mixtures, but not a street-tuned normally aspirated engine.
Lot of cruise controls will basically floor the gas pedal to accelerate. And flooring it is about the worst thing for FE. Do the acceleration yourself, slowly, then set the cruise control at the new speed when you get there.