I've seen a post or two about this topic before, but I unfortunately haven't been able to find anything I'm too certain of.
I've been driving my '09 sport around with an overall average of 19 mpg. 24 is the highest I've ever gotten, 14 being the lowest.
I drive mainly in the suburbs, so there's often a lot of stop-and-go traffic which might be a part of it. But this is still really low compared to what I'm hearing on here.
I have recently (within 6 months) changed:
- Spark Plugs
- Coils
- Oil
- Tires
- Engine/Cabin air filters
I usually drive with no load whatsoever, and my driving style is admittedly sometimes on the more aggressive side (as in how sharply I start and stop) but even when I drive very gently, I don't go too far above 20.
No issues with the engine currently to my knowledge, though maybe it has something to do with the spark plugs and coils? I think I bought cheap ones from Autozone but I'm a little skeptical that they managed to just kaput my mpg, plus it was the same before I replaced them (though to be fair I had a few misfires)
Maybe the plugs are loose, but could they really get so loose after just half a year?
If anyone has any ideas or comments on this, it would be much appreciated.

 

city/suburban driving is super subjective and makes it hard to figure anything out
I've not had a tank below 31mpg in all 40K miles of driving (that was a tank of prettymuch all "around town" driving)

19-24mpg is what I averaged with my modified 2005 GTO (6.0L V8) tipping the scales over 4000# and just over 600HP/TQ


you may want to do a highway test,
also check your display milage against true milage (fillup to fillup, Miles devided by Gallons used)

Fill up the car, reset the trip meter,
drive conservatively to a true highway, get to 60/65mph and set the cruise, go for 50 miles or so (the further you go, and the better the info you'll get), and come back.
fill it up again, verify the mpg displayed matches your math

at 60mph, on flat ground, I'd expect between 35 and 40mpg
I average 37mpg on my commutes (but I'm going 60-70+ and have lots of hills, as well as 5-10 min of stop/go traffic on one end)

Good luck

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For fuel efficiency, you basically have the vehicle, the driving conditions, and the driver.

1) Vehicle - it sounds like you've covered most of the bases here. Stuff like: properly running engine (e.g., clean air filter, clean oil, good spark plugs), good alignment with properly inflated tires, properly adjusted brakes that aren't dragging, no unnecessary weight in the car. Based on your description it sounds like your fuel efficiency is pretty steady before and after maintenance, which argues against a problem with your car. Can you confirm that your car doesn't have any trouble codes?

2) Driving conditions - Stop-and-go driving will certainly decrease your mileage vs a highway trip at constant speed. Winter temperatures, winter road conditions (often poor), and winter gas will decrease fuel efficiency. Can you confirm that you get the same mileage during the summer and winter? High speeds also lower fuel efficiency, but if you're mostly doing suburban driving wind resistance shouldn't play a big role for you.

3) Driver - When folks complain about their mileage and don't say it's a new problem I lean towards the driver being the issue. What sort of fuel efficiency have you gotten in other vehicles you've driven, and how does it compare to the expected fuel efficiency for that vehicle?

I will just use myself (2010 Fit Sport A/T) as an example. I can't imagine how to drive a Fit in a way that returns full-size pickup fuel efficiency, which is basically what you're getting. Even during the pandemic when our only driving for several months was a weekly 4 mile trip to and from our favorite grocery store in the hilly city we were stuck in we got around 30 mpg. My lifetime Fit mileage is 38 mpg and on long trips I normally sit in the low 40s.

If you want to do some testing, I can think of a couple options.

1) Next weekend go for a long highway drive. Fill up the tank, drive for 200 miles at a steady speed around 60-65 mph (use cruise control if you have to), refill the tank, and calculate your mileage. If it's not at least in the mid-30s you have something wrong with your car.

2) Rent a small, fuel efficient car (Nissan Sentra, Kia Forte, Toyota Corolla,...) and use it for a week of your normal driving. As above, fill the tank, drive for a few days, refill the tank and calculate your mileage. If you get similar mileage to your Fit in a different fuel efficient car then you're most likely the problem.

3) Loan your car to a friend (preferably a friend who was not a background character in a Fast and Furious movie) and have them use it for a week. Track their fuel efficiency. Is it better than you get? If so, this again points to you being the problem.

 

Little update here; MPG problem solved(?)
After looking at the replies, I decide to go on a tiny 'lil road trip to measure the true MPG. Half suburbs, half highway. Before departing for the highway I topped off my fuel and reset my trip meter, and after traveling pretty briefly, I did average about 36 MPG according to calculations I later made at the pump. I reset my trip meter again and decided to just drive around a little. I ended up averaging 32.4 MPG (according to the car's computer) by the time I got home.
So clearly my car is capable of getting a normal MPG, so why wasn't it before?
I have three factors that may play a role.
1. MAF sensor. I literally unplugged it while the engine was running and stalled my car, and I swear to god after I plugged it back in and started the car again, it felt ever-so-slightly smoother. I might be tripping, though. (Note that this is a legitimate way to test the MAF sensor, as ridiculous as it sounds. If the car continues running after unplugging it then that means it's toast. The only real downside to this method is a CEL popping up, but this went away for me within 12 hours.)
2. My driving style. As I mentioned above I do start and stop quickly usually. This time I was much much more gentle with the gas and break. (Though I have done this before and it usually doesn't make much of an MPG difference)
3. Driving conditions. I was driving in much less stop and go traffic than usual, and my car was already warmed up before the trip and there was less idling and blah blah. My normal commute to work takes ~7 minutes and consists of me starting my car, driving for a bit, getting to two annoying intersections that are always red when I get there, and then eventually pulling into work. Perhaps not driving in such a stupid cluster of traffic also raised my MPG.
All in all, looks like my car is not the problem. I'll try being more gentle when driving to see if it keeps my MPG up. If not, then it's probably mostly the third factor.

Regardless I'm just happy I don't have to replace anything. I'm a bit of a freak when it comes to car maintenance. I thank you KrautBurner & Drew21 for your responses which pushed me to take my Fit on the highway

 

Also there is a TSB / Software update for the average MPG display in 2009 Fits. It reads 10-20% high in my experience, so don't rely on it as a source of truth, unless yours has been updated.

 

Based on your more detailed description of your typical short (time/distance) drive with potential prolonged idling at lights, that definitely would play a role in poor mileage. Especially in the winter when your car might not be fully warmed up until most of your drive is complete.

As someone else noted, the mpg calculator on the dash can be off. I always calculate efficiency at the pump and compare with what my car is telling me. The funny thing is sometimes mine is dead on (within 0.1-0.2 mpg) but other tanks it's +/- 2 mpg from the pump calculation. I have never been able to discern a pattern (e.g., long trips, more local driving, etc). It doesn't really bother me since I rely on the pump calculation for my records.

I would suggest that the more useful tool the car provides is the sliding bar graphic showing instantaneous mileage. In reality, the numbers on the scale don't matter as much as the easily interpreted bar (low = bad, high = good) that you can see in your peripheral vision to monitor how your driving behavior is affecting your efficiency.

 

Yeah I'm aware the car's computer is often off. From my experience it does tend to be more accurate though, but regardless I usually go to it for convenience. When trying to get a very accurate reading then of course I calculate at the pump.
The little sliding bar showing instant mileage has always been something I've loved about my Fit, seeing it skyrocket to 80+ when I would drive downhill fills my brain with dopamine. I should probably use it more to note my own driving habits, like you said. Thank you )