[olddog]

I would not rule out brake drag. Yes when they drag hard they get very hot, but they can drag lightly and not get hot enough to notice.

My 98 Ranger slowly dropped to 18 MPG. It had been at 18 MPG for several years. Over 200K miles, I just figured the O2 Sensor was off, since it was never replaced. One day someone stopped hard in front of me, I realized the back brakes were no longer working. The cylinder were frozen and the lining had come off the shoe and was dragging. Everything was rusty from the heat but I had never notice any signs other than the MPG. After fixing the brakes, the MPG was back up to 21.

My son's 2010 F350 diesel was down a bit on fuel mileage. He had a front caliper stick while towing his camper a couple turns from home. He got out his IR Gun and all four wheel's were way hotter than he expected. Turned out they were all dragging. Replaced the calipers and fuel mileage jump back up.