After having the oil changed my MPG was down to 16.2 instead of it’s normal 20 MPG. I took it back to the mechanic and they did not have the hoses hooked to the engine air filter. Idiots. They hooked them up but the MPG still remain really low

 

The ECU will adjust and your mpg will eventually return. You can also take the negative terminal off the battery to reset it. For the first bit, the mpg will be bad as the car will dump in more fuel at idle, etc before it re-adjusts.

 

Well now after filling up with gas the mpg read out went from 16, 22 ……30 and finally down to 16.5. What could be happening to my car?

 

Do you notice the same trends if measuring manually? I find the MPG readout on my car inaccurate.