So my 2008 accord 4cyl randomly dropped in mileage. I recorded 13mpg city and 19 hwy. That’s a huge drop from 19 city and 27 hwy that it was getting. there’s no check engine light on and the engine runs exactly the same as it did before. No acceleration or idling issues? Anyone have any ideas? Please don’t say change the oil or inflate the tires because those are already done.

 

If you want to throw a part at the issue versus paying someone to diagnose the issue, change your upstream O2 sensor; if you opt to go this route, use either NTK or Denso sensors, under no circumstances should you buy a Bosch sensor, they really don't work well in Honda products.

 

Have you ruled out dragging brakes?
That's a huge drop in MPG, so if the brakes were dragging enough to cause that, they would get pretty hot. But still, have you checked?

 

ive been driving it with the mileage issue for a few months and I haven’t noticed on braking issues so I don’t think it’s that

 

Easy to check; and you'd be surprised how bad it has to get before you notice it by simply driving.

 

how would I check something g like that?

 

and anyways, the car still drives and feels the same way it did before. Only thing that’s changed is the mileage...

 

When an O2 sensor starts failing, it often will not fail enough to cause any engine lights, but the engine will still be running rich. Given gasoline engines are throttled by air, as long as there is enough fuel for the quantity of air, it will maintain power, running even richer will cause an almost unmeasurable reduction in power, but a huge reduction in efficiency.

 

After a good drive, walk around the car & see if any of the wheels are noticeable hotter than the others. But realize the fronts will be hotter because they do more braking work, and because of the engine heat. This is a better test if you already have a good idea of how hot they get "normally".

Another easy way to check is in a flat smooth parking lot. Get going slowly, about walking speed. Shift to neutral & coast, paying attention especially when it comes to a stop. Dragging brakes will make it lurch a bit right as it stops.

But the best way to check for dragging brakes is to lift each corner & spin the wheel.

There's other things that are less likely, so if neither the O2 sensor or the brakes are the culprit, come back for other things to check. It's a process of checking & ruling out each idea.
- tire pressure - you already checked.
- oil change - done; but does the engine use any oil between oil changes?
- engine developing more blowby, indicated by oil blowing back thru the vent into the air intake duct?
- clogged catalytic converter? (but I imagine you'd notice a loss of power)

I've often seen a dirty air filter as a suggestion but don't understand that. Unless it's an old carburated engine, a dirty air filter is equivalent to not opening the throttle as much. Probably good for MPG while limiting the max power.

 

Your understanding is correct. The EPA funded a study, I believe by Oak Ridge National Laboratories, a few years ago where they took something like for modern OBD-II fuel injected cars and one old carbureted car (an old Pontiac Grand Prix if I remember), and ran then with all grades of air filters from new through horribly clogged (like from over 100,000 miles of driving in a very dusty environment); the only car which showed worse fuel economy was the old Pontiac. The others either showed the same fuel economy or slightly better (probably due to the power limitations).