I have a 2008 Chevy Impala LT with a 3.5l.
I have 83,000 miles. At about 80,000 miles, the display
went from 26.3 MPG to 20.9. There are no problems and the car runs fine.
The only reason I noticed was went I filled the tank and the mileage till empty went from 425 to 360.
Anyone have any ideas?
Thanks,
Dan

Welcome to the Forum Dan! There are so many factors about MPG to consider.
In general Mpg will vary based on users speed/city or hwy, heavy or light foot driving, mountain or level road conditions, complete state of tune, clean oil, proper tires & inflation, & removing excess weight in trunk or heavy passengers... LOL.
At 80,000 miles new spark plugs and air filter/ full tune up should be considered if one has not been performed. Other things I do to improve mileage are clean or service the throttle body. I also over inflate my tires by 3psi to get better mileage as well, based on door stickers recommendations. I also look for symptoms of brake drag and wheel bearings as possible causes of poor mileage if all the other things are up to speed.
Idle times =0 mpg and cold weather warm up times wont help either. It takes longer for car to warm up as well.

could have gotten some bad gas. thats about what mine got when i put e85 in it, try getting gas somewhere else and checking the other things art suggested