My gas mileage has fallen recently by a lot. The car seems to be running well/normally but mileage is very bad. I want to say 8-10 mpg? Plugs are good/Air filters are new. What is going on?

Also, unrelated issue -- starter occasionally doesn't immediately crank when I push the button. About a one second delay... Starts fine, but it didn't do this before...

Thanks for your thoughts!

 

Gas mileage is either failing cats or O2 sensors.