Your fuel economy sounds rather low to start with. Regardless, if the problem started after changing the plugs, I'd bet on a bad plug or loose connection.
Be sure all the connections are secure.
If so, try disconnecting one at a time so see if you can isolate the problem to a particular cylinder.
Presuming you can, maybe you have a bad plug or a bad coil. At 9 years old, it could be a bad coil, though mine are still OK. (Don't jinx me!)
FYI, OEM plugs should
come pre-gapped and if you messed with them that could be the problem.
From what I've read on other posts, .44 seems to be right, but there is conflicting info out there. DO you have a decal under the hood? Have you checked the owners manual?
Here's a thread I just checked: Spark Plug Gap question
Oh, I just realized this is your first post. Welcome to the forum!
Use the search window at the top right of this page and see what else you can find.