I have a 2001 F150 SuperCrew (I need the extra car seat room for all the kids) with a 5.4L v8. It now has about 68Kmiles. Over the recent XMas break, I replaced the plugs with MotorCraft double-platinum, same type & number as the original stock plugs. Over the past few months and about 6K miles, I have experienced about a 2 MPG decrease in gas mileage, which makes no sense to me. I have no noticeable decrease in performance. I have kept records of every gas receipt (and every other receipt) for this vehicle, and graph them periodically in Excel. (yeah, SWMBO says I'm really really AR - I already know this...) Also, I do and have been purchasing gas at the same place for years - Texaco then Shell 93 octane) As far as I can tell, there are no vacuum leaks, and I have a new air filter, PCV valve, etc. The engine is completely stock.
Most driving is in town, and I was getting about 13.57 MPG. Now, I'm getting 11.48 MPG (smaller sample obviously, but a clear deviation in the pattern as compared to the past several years)
Anyone care to speculate as to what might be going on here, and what might be causing my sudden decrease in gas mileage? I suspect this is somehow related to changing the plugs, but the replacements are the same as the originals and there is no "miss" or anything else I can notice/find.. I would appreciate any suggestions as to what/how to look for my lost gas mileage.
Thanks,
++ Dave