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Update: It was very likely a bad O2 sensor. I took it to another mechanic and he told me the same as the dealer that the computer didn't show anything wrong. One of the things I found consistently when searching online was that a bad O2 sensor could cause a 20-30% drop in mpg. Since that's what I was seeing I asked the mechanic about it and he said it was possible. He said if I wanted to change it out to buy the part and bring it back in and he'd do it for me.
He just finished and showed me the old one and said it was running rich so that was probably the problem. I just filled up and I'll know in a couple weeks when I go through this tank if that fixed it.
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