Hello All, I'm browsing around past threads that address low MPG troubleshooting, will post where more appropriate, once lowest hanging fruits have been addressed.

I've got a 4 cyl 6th gen 4 door USA accord, and it gets 19.7 mpg now but used to get more like 25 on average when i first bought it in 2015. I purchased it with after market o2 sensor (on the header at least), and the cat was fouled, tailpipe had soot.
Tailpipe is clean now, it idles a little low (700 rpm in drive at red lights, but otherwise the motor sounds decent (i'm learning how valve lash sounds like)). I changed the spark plugs, and the gaps were way too wide, but the new plugs did not improve mpg. Did not change the spark wires.

I'll be checking the EGR for heavy soot, and cleaning fuel injectors. I've been told the fuel pump has a fairly coarse 100 mil filter and shouldn't really clog. Looks like other similar cases recommended checking the o2 sensor on the exhaust header, and even the fuel line for leaks. I'm in California and we've got at least 10% EtOH in the fuel, though i don't really smell gasoline ever.

My personal guess though, it needs a valve adjustment, and secondly, since the auto tranny is failing (has been for years i just feather it), that could induce power losses-- though by 20%? That's crazy.

Tbh the car is a **** box and not worth any effort into repairs unless i get a good deal on the tranny, if only the fuel consumption was @ 25mpg. Honda is known for motors, so there's gotta be something I'm overlooking for these 21% mpg losses.