Sudden fuel mileage drop

[Beetle77]

1998 diplomat Cummins 24v 5.9
I make the same trip to Orlando every other month, filling up in Georgia on the way down and in Orlando on the way back back. I don’t know the numbers but going by the gauge my fuel consumption has been consistent on every trip. On this trip I filled up in Florida and used a quarter tank more on the way back, it seemed like power was down a little, taking more boost than usual on the hills. The only difference was that this time I filled up with diesel efficient fuel at Mobil. The engine was also running 15-20 degrees hotter than usual but not in the overheat range .fuel filter and air filter were changed before this trip, also it was at night so I couldn’t see any exhaust color change.
Any ideas?

 

["EZ"]

Look at your air filter. If it's plugged it will make the engine strain to breathe and kill your fuel economy. If you got a bad tank of fuel that may also cause the MPG drop but should be evident by the engine running poorly.

 

[dbarnes]

You might have your Cummins checked out for a EGR issue. If the engine is recycling exhaust gases too much it will lose power and cause elevated temps. it also destroys the fuel economy as a result.
other than that, worn injectors that seep fuel. Start it up in the daylight and look for excessive white smoke of unburned fuel.
Of course sometimes it is simply headwinds which are harder to notice at night.