Car engine runs smooth but gas mileage suddenly is 5 mpg less
Anything that sudden, probably requires a call to your mechanic for a checkup.....
Is the check engine light on? If so get the ECU scanned for codes at a local auto parts store and post the codes here. A bad O2 sensor can do this, but it will usually illuminate the check engine light.
I had a customer's Crown Victoria come in for a very similar problem, after running a fuel pressure leak down test found the injectors were leaking slightly, fortunately a good cleaning fixed the problem and I did not have to replace the injectors, so try getting the injectors cleaned and see if that helps. If not then you can move onto having the vehicle live data scanned to see if some sensors are slightly out of specs but not enough the turn on a check engine light. HTH
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