My wife drives a '96 Sable on her 40 minute daily commute. Almost all the miles on each tank are highway, yet she’s getting 21 mpg when she should be getting closer to 26. She says that relatively recently it has begun to run “fast” as in the tach runs about 500 rpm faster than she thinks it used to. She says this happened relatively quickly, possibly even an all-at-once change and not gradual.
I don’t drive the car enough to say if it’s running faster than it should or not, but it certainly is not getting the kind of mileage it should. I’ve asked her to make sure the Overdrive is not disengaged this morning, but I haven’t heard yet. That’s all I can think of though. It’s got 177,000 miles on it. Anyone else run into this sort of thing.
It could be not going into overdrive. Check the automatic transmission fluid level. If the automatic transmission slips a little, sometimes, it could help to change the transmission fluid and filter.
Perhaps the torque converter is not locking up.
Sounds like you may have hit the wall where the performance of a new car has been lost. There may be no way to get it back. Winter will reduce engine efficiency too. The Taurus/Sable with that many miles isn’t a rolling miracle, but it’s time to consider getting your next car.