Just verified with the invoice that Dexron VI (full synthetic) was used.

Replaced wires and cleaned MAF, increase of 1.5 mpg now. Still strange but that’s progress.

Will do! I’ll let you know how it works

Oh absolutely I agree. But when you do maintenance and the result is a 25% loss in fuel efficiency, there’s something awry. Driving habits have not changed.

While this is really solid advice, I did not disconnect the battery on this job. It’s been 400 or so miles since the maintenance and it’s staying around that 12 mark.