Significant MPG drop
 

I have a 4cyl 2006 Honda Accord SE that has, oh, about 135,000 miles on it. The car has had zero issues until now. The only things I have ever done to the car are routine maintenance and the replace of all the belts awhile back. I use Mobil 1 full syn motor oil and OEM plugs. For whatever reason, the car now gets about 350 to 360 on a tank of gas whereas before it was more like 390 to 420 on a tank.

I did have the the upstream O2 sensore replaced, as my trusty mechanic felt that this was the culprit more than anything else. Well, it wasn't.

There are no indicator lights on, so the computer is not reading that anything is a miss.

Winter blend?

I get a drop in mine this time of year. I go from 32.X MPG to 28.X MPG....or to put it into "per tank", I like talking MPG but - from ~360 to ~320 per tank. Average fill up gallons are in the ~11.5 for me.