140k miles on it. I changed the coil packs on it, changed the air filter and cabin air filter, tire pressures are normal, spark plugs are still good. The only thing that I haven't done yet is change the transmission fluid but I'm going to do that tomorrow. What could be wrong with my Fit?

 

Have you not doing your valve lash tolerance.yet? If not, diagnosis is complete and go get that done. Your valves drift out of spec and will tank your mileage. In the Fit's case the exhaust valves tighten over time and stay open too long and let unburned fuel and air past the cylinder.

 

Agree with checking the valve clearances given the number of miles. Also not replacing the plugs with 140k miles if they were original is not a good idea IMO. Where you getting cel codes for a misfire(s) and then decided to replace all the coil packs ? Valves out of adjustment can produce that code, but not enough info given, so just giving suggestions.

 

Calculate MPG manually with pen and paper for a few weeks to confirm.

Agreed with new spark plugs and valve adj. Also see posts here on updated torque specs. Valve adjustment is 100k miles per Bernardi Honda; the exhaust valves can bind up.

Check tire pressure. I don't think wheel alignment or balance would cause such serious mileage drop unless you have serious issues.

Get good fuel from a different top-tier vendor.

Check that you don't have a binding brake, binding emergency brake, wheel bearing issue.

You might want to do a voltage drop test on the electrical system and clean the main ground and power contacts.

You might consider cleaning the air intake system etc. This makes a big difference in how our other cars drive. These are the steps we plan on doing to the Fit.
- New air filter
- Clean throttle body & IAC valve (caution as don't want to spray off moly lubrication at butterfly valve)
- Clean MAF (only special MAF sensor cleaner, don't touch internals)
- Clean EGR valve and passages (very easy access on Fit and common issue apparently)
- Replace PCV valve

https://www.fitfreak.net/forums/2nd-...-question.html
"www.youtube.com/watch?v=e-PzkoGhv0M" Honda EGR Valve Cleaning DIY VIDEO
"www.youtube.com/watch?v=v1xdHMWSFNM" Cleaning 2008 Honda Fit EGR Passages VIDEO
"www.youtube.com/watch?v=h1cHn_ld2C8" HONDA CITY/JAZZ Cara Membersihkan Throttle Body dan IACV VIDEO
"www.youtube.com/watch?v=cwo1Hs_0aSI" Honda Idle Air Control Valve, PCV and Throttle Body Cleaning VIDEO

 

Being in California you also need to take into consideration that they might have already started mixing more ethenol into the gasoline to make the winter blend fuels.

 

I'll get my valves adjusted, thanks guys.