12 city and 20 hwy, what is happening?

With that kind of mileage you could be running a V8. I’m thinking the actual engine temperature should be checked, it maybe running too cold, stuck open thermostat so the computer thinks it needs to run a richer fuel mixture trashing the fuel mileage. No a dealer will not check this out cause diagnostics is a real money maker for them.

check the simple stuff.. air filter and MAF. air filter as clean as possible. Have you sprayed your MAF with cleaner?? DO NOT touch any part of the MAF while you spray it ( wear gloves) and let it air dry about half hour and then assemble back together. Try that for starts. I did that to my VW 1.8 t and I improved to 35 miles to the full tank

Also, if you have a tachometer, keep an eye on it. Especially when cruising on the highway. If the tachometer is over about 1,500 RPMs when cruising the transmission is not shifting into overdrive. That will cause poor gas mileage. Hope that helps! Jim