So I purchased a 97 sunfire as a commuter vehicle to drive back and forth for college. It’s the base model 2.2 5 speed. It’s got 184,000 miles. But it’s been kept up great. There’s no leaks no oil usage. Nothing. The only thing is the fuel milage rating for the car is 33 for the highway. I’m on the highway speeds at 60 for 90% of my driving. And the best I’ve gotten is 25. I’ve got a code that pops up from time to time for the rear oxygen sensor but it comes off within a hour or so. But the downstream oxygen sensor doesn’t effect fuel efficiency, right? I’ve heard mixed things on that. I plan to do a full tune up using only AC Delco parts. Spark plugs, oil and filter, fuel filter, etc...would anyone have any kind of a guess at to why the milage is so far off? The car runs great and has plenty of power I just don’t understand why the fuel milage is so far off it’s claim. Any help is appreciated.