I had the plugs replaced in my 2012 Cayenne S. No other work performed that could be related. No change in tires or tire pressure. Immediately after the new plugs were installed my gas mileage dropped from a highway average of about 20 to a consistent average in the 17s. Been consistently about 12% lower mileage now over about 2500 miles. Took it back to shop - they say: correct plugs and the pre-set gaps are correct and they are stumped.

Anybody have any experience like this?
Thanks much,
Ken