My Fit 2009 used to get 30mpg now it's averaging 25-27mpg
 

It's at 135k now. I've changed out all the air filters, the spark plugs still look good, and I added fuel injectors into it already. What could be wrong with my fit? Is it just old age? Also the last time I did an oil change I used 5w-30 instead of 5w-20 (my mechanic told me to do that due to the wearing of the engine pistons, they need a thicker oil).

Tire pressure?

Has your commute changed? Do you have winter blend gas in CA? It’s been cooler out so has the car been idling more? Tire pressure? 5w-20 is a more fuel efficient blend then 5w-30.

also check your air filter if its dirty mine was doing the same now im back at 34 mpg

It might be the 5w-30 then because I notice this started during the summer too. Probably around the time I changed my oil to 5w-30. I'll monitor it from here because I just did another oil change and I switched it back to 5w-20.

Yup, changed all my air filters.

I doubt it's that. My tire pressures are always at normal.

have you cleaned the air sensor? and around the throttle could be dirty also i clean my egr as well

How old are the Ignition Coils?

I never replaced it.

Could be part of the issue. Often the plugs will last longer than the coils.

Same thing happened to me, didn't think much of it. I am at 62k miles. Finally got a misfire code for cylinder 2, turned out the spark plug in cylinder 2 was backed out and cylinder 3 was halfway out. Changed all my coil packs, plugs, and did a valve adjustment and now I'm averaging 35mpg my last week of driving. Car is smoother and peppier as well. Check your plugs and coil packs, it took the plugs backing all the way out to throw a code, yours may be loose but not enough to throw a code just loose enough to bork your mpg.

im at 200k and i haven't adjusted my valves im too scared to do it myself im in socal

Its not a scary job, just a methodical job. I'll link a video at the bottom on a DIY. Requires very basic tools and a replacement valve cover gasket, which Honda wants about $9 for. But, a valve adjustment isn't going to net you any significant mileage increase. Maybe 1 mpg if they're way off but you'd be lucky to see that big a jump.