Yes, kudos to 360View for explaining the incredibly complicated world of MPG.

Re: spark plug change. In this particular case, yes, even though I don't believe it really happened. But if it did, the mpg increase resulted from an increase of combustion efficiency. If it did happen, you probably had one or two plugs that weren't firing all the time. Getting those cylinders to fire all the time with new plugs increased power and mpg.

Without changing ANYTHING ELSE, increasing combustion efficiency will always result in an increase in power. Always.

R.