<img src="/forums/images/graemlins/confused.gif" alt="" /> I'm not following you here...

You still haven't mentioned what type of tires. And for that matter, what air pressure are you running. I realize this is not the "fix" you're looking for, but MPG is such a wide-open thing, and there are many factors which can contribute... If your mileage dropped to 2/3 of what it was, then it could indicate a problem, but it could also indicate a change in driving habits, terrain, equipment, etc.

But all things being equal, and assuming everything else checks as "normal" when was the last time, if ever, that you had your fuel system serviced, ie throttle body cleaned, injectors flushed, etc? This can affect gas mileage, but its typically a gradual decline, rather than a sudden drop...