I changed spark plugs, valve adjustments, put in new fuel injector, changed air filter and cabin air filter. Don't know what else to do. If I brought it to my mechanic would he be able to figure it out and find a solution and resolve it at a reasonable price? This has been an on going issue for a few years now and I still can't figure out what's wrong with my Fit.

 

Are you saying that you've been getting 23 mpg in a Fit for several years despite keeping up on maintenance?

This sort of suggests that the issue could be the driver, or at least the driving conditions and patterns.

As an example, in early 2020 I found a mint 2010 Fit Sport (AT). My dad and I did all the recommended maintenance and then I drove it about 1500 miles back to where I live. On that highway trip I averaged 41-42 mpg (calculated at the pump). Then Covid happened and my wife and I only used the car 2-3 times per month to visit the grocery store, with each drive being about 7 hilly miles (roundtrip), stop light to stop light across town, or what I would consider the worst possible conditions for fuel efficiency. During that period I averaged around 30 mpg (bit lower during winter, bit higher when warm).

What are your regular driving conditions (highway, city, mix, hilly, flat)? What is your driving style (Fast & Furious or Driving Miss Daisy)? Auto or manual transmission? Do you keep your tires aired up? Any indicator/warning lights showing?

Are you calculating your mileage at the pump or using the meter in the gauge cluster? Mine is always different from the pump calculation, but only by 1-3 mpg and the direction seems to change randomly.

Most importantly, what sort of mileage do you get in other vehicles when driving in the same conditions as your Fit - lower than expected or higher than expected?

If the Fit is the only car you have driven where you get (well) below average mileage, that would suggest there's a problem with the car. If you regularly get poor mileage across a variety of vehicles and driving conditions, that points to you.

 

You didn't say...but it sounds like you have an auto tranny. 23 mpg is low...even for the A/T
My 2010 never gets less than 38 mpg between oil changes. It's a M/T and has 240K miles on it...and I wring it out between gears.

 

I believe I keep up with maintenance. Oil changes, atf change, tire pumping, tire rotations, everything listed in my OP. I drive reasonably safely so I don't speed. 50/50 highway and city. Auto tranny.

I will try check my mileage at the pumps cuz I haven't done that yet...

 

I feel like I say the same stuff in every thread
Not that i think it's likely, but check brakes and alignment, stuff that can cause rolling resistance (like significantly under-inflated tires) if you're confident in the previous maintenance.
two ways to increase performance of this kinda thing. more power, less resistance!