My MPG display isn't making sense most of the tiem. Seems to get stuck on 11.8 MPG especially when tank is 1/4 tank or so...

So how does the system compute MPG---what's mine doing wrong to get this result. Seems to have happened since I changed spark Plugs ???

 

I believe the PCM estimates fuel consumption by how much air is being taken into the engine. It can be spot on sometimes, and very far off others. Your results may vary.