Somebody who experiences an mpg increase from 21->28 under the same driving conditions was not driving in an economical manner in the first place.
Some of the veterans of this board will probably recall me talking about my nutty ex-boss who had the habit of turning to look at his passengers when he spoke to them...which was...often. As a result, everyone hated riding with him because of the constant whiplash effect. (Jerry hits the brake and everyone's head lurches forward. Then, he hits the gas again, and everyone's head lurches backward.)
Anyway, Jerry complained about the awful gas mileage that he got with his big Buick, so--for two reasons--I suggested that he start using his cruise control. I made the suggestion because it should have improved his gas mileage to some extent, and because I thought that it might possibly stop him from doing the gas/brake two-step constantly.
He complained that he hated cruise control because he had to keep re-activating it. Of course he did, simply because he couldn't break himself of the habit of constantly hitting the brake, and then compensating by hitting the gas pedal.
Note: This was the same guy who always bought a specific dollar amount of gas--usually $5--instead of filling his tank, "so that they can't cheat me".
Yes, he had to go to the gas station every day into order to get his usual $5 worth of gas.
Yes, he was a very strange man...