Recently, my 2012 G37's MPG goes down from 20.7 to 19.6. Everything is the same as before. I thought it might be the battery issue. It doesn't get better when I replace the battery. Is anyone know what may cause this issue? My car runs around 48k miles now. Do I need to replace the spark plugs or change the transmission fluid?

 

1 mpg concerns you? Is this real? Could literally be anything. Even gas stations or winter blend ambient temp oil life oil age etc. 1 mpg should not concern you enough to start replacing parts.

 

 

If you live in a major metropolitan area, you are subject to EPA gasoline blend changes. This alone can knock mileage down 10% or more. You didn't provide your driving style or distance driven to judge whether that has had an impact on your fuel mileage. Did you buy new tires? Have you kept up with inflation pressure after winter set in? Changed oil or oil brand or type? If you have a 6MT, maybe your clutch is starting to slip. Ad infinitum.

[Edit]I just noticed I assumed the OP to be a U.S. resident. Silly of me.