On the highway, you should be somewhere around 35 mpg at an avg speed of around 70. I get 34.5 in fifth...ymmv slightly...and should be slightly better.
Start with thermostat...bet you're showing a little to the left of dead center on a cruise. Shows it's broken open and will rob mpg.
Fuel filter also will take mpgs.
Then other basics, maybe ignored...air filter, plugs (at 100K miles), and then, there's the odd malfunction that seems to hold the brakes on...e-brake sometimes binds...(some odd ball things like this happen rarely, but they happen).
Start with basic tune up stuff and go from there. Get codes read too...oh, and you could have an air leak too...almost everyone does...and when the car sees it's running lean, it adds fuel...lower intake boot is one common leak...but check as many vac lines as you can...look beneath cabin air filter housing behind intake manifold.
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