When I first cold start it every morning it runs perfectly no problems. A good 20 minutes pass and it starts misfiring and the whole car shakes. It shakes at a point where the steering wheel shakes also. I also hear a tapping sound when I first start it up but goes away and very faint after a 5 minute drive. If it helps I get 13.8mpg now.. a week ago I got 17mpg. Is it the spark plugs or something more major?
My car has 294500 miles.
My car has an automatic transmission.
It is possible that you have an ignition coil going bad. Once heat builds up, resistance in the ignition coils increase. If you have a weak coil, it will sometimes cause a misfire when it is hot.
The misfires are why your fuel milage is worse. With two cylinders not making power, it may take more fuel to produce the required power. The tapping sound may be caused by incorrect valve adjustment or other valve train related components. A certified technician, like one from YourMechanic, will be able to diagnose the cause of the misfiring issue and assist you with repair.