Why is my 06 impala not getting gas mileage that it did before

If you haven't done a tune up on it recently oil change filter, plugs etc. That might do it or check your tires for poor tread or also poor inflation that's my best guess

Dirty sensors, throttle body, egr, pvc, air filter, fuel rail/injectors, aged/plugged catalytic converter, etc., can all impede gas mileage. On the more serious side, mechanical wear on the engine such as gunked lifters, burnt/worn valve seats, worn valve guides, loss of compression from worn piston rings, worn/slack in timing chain affecting timing, valve lash, worn valve springs, worn cam lobes, worn bearings... there's a lot that can affect gas mileage, lol. But keeping the car well tuned and maintained should keep the gas mileage up where it was when new. If a tune-up alone doesn't restore some gas mileage try degunking the engine as well with some Seafoam or equivalent.