...an average of 14.3 mpg is ABYSMAL. Please confirm for me, anyone, that this warrants having my vehicle serviced. It’s a 2016 Touring FWD 2.5L.
"serviced" is or could mean lots of things.
Be careful just taking it to the dealer as they will want to pile on a lot of unnecessary services that do little to help you, but do more to help their bottom line.
start with simple stuff. How old/many miles is the engine air filter. If several years old, OR, extremely dirty (like in a high dust area), change the air filter. Please stay away from the oiled K&N filter or simillar. They provide no real advantage to a standard paper filter and can actually cause more headaches...plus paper OE/OE style filters are cheaper.
Have you thrown injector cleaner in the tank to ensure the fuel system is "clean"/"cleaner"? The injector cleaner might drop mpg 1 or 2, but is just more more proactive step in checking off each system.
Have you checked your ECU for trouble codes? Not all codes throw a CEL. Scan your ECU with an OBD-II reader, or take it to a big chain auto parts store, like Autozone, Advance, O'Reilly, or similar and see if their scanner shows any codes. (if codes exist, take it back to the engine installing dealer)
Have you looked at the spark plugs? What condition are they in? Are they still gapped properly to spec? Is it a good time to replace them anyway?
What oil are you running? 0w-20? 5w-30? How old is the oil? Does it meet manufacturer spec (see owners manual)?
Are there any vacuum leaks? Lots of little places for leaks to occur, especially if the below action just happened VVVV .
I forgot to mention that I had a new engine installed at a Mazda dealer’s service department on 5/31/22. I also put 4 brand new tires on the car on 6/1/22.
You said "new engine". Was it a new engine, or one from a parts yard (or low mileage Japanese import)?
And from what year was this engine; same as the car (2016), or older, or newer?
Is there a warranty on that engine or the labor/install?
How many miles/kms on the "new engine"?
It is possible the "new engine" could need a valve cleaning service, aka walnut blasting? Check the tops of the valves first before getting the service done. A lot of garages just use chemicals through the intake to clean the valves vs blasting as the chemicals are "cleaner" and create less of a mess; plus require much less parts removal than the walnut blasting.
I want to ass.u.me, the installing dealer did a relearn on you throttle body...again, an ass.u.mption.
On the new tires, what type/brand? Some tires have more rolling resistance than others, but not so much that a 6-mpg hit is caused. Is the alignment good? Too much toe (in or out) will cause extra drag and hurt your mpg. Again, making an ass.u.mption that the alignment is good (within spec).
What air pressure are you running in the tires? I run 10% over what is on the door sticker; somewhere around 33-34 psi and I'm on Nokian WRG4 SUV tires.
Next up would be the O2 sensors. pull them and take a look. what color are they? Replacing them isn't horrible, but not cheap either. I check prices against RockAuto and see if there are any advantages to buying local vs buying on line, paying shipping, and waiting.
Do you think you have brake drag? how do the brake pads look? yes, I'm throwing out every possible thing I can think of that would/could cause an engine to perform poorly or cause it to work harder, like extra friction/drag.
Getting to a solution could take some time, and could Invlove swapping parts that may or may not improve the situation. but once a part is replaced, you know that is good