My 2004 SXT Neon had piston slap since day 1 like you described... it's pretty common on the ol Neons. Most I ever seemed to get out of it was around 38+MPG myself. Sounds like you are on the path to exceeding what I could pull out of it! Especially since mine factored in frontal blocking. Good job on keeping the ~11 year old running like a champ!
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