I've had this Honda for a while just reached 15k miles. It's a 4 cylinder
I noticed that the MPG has decreased badly, I haven't really had the money to fill it up all the way and I only use it about 2 times a week for school,but from the last 3 times I have added gas it has been decreasing.
The first time i started checking I put in $20 and it got about 4.3 gallons and it gave me about 70 miles which is about 16mpg

The second time I put in 20 which was 5 gallons and it have me 90 miles which is 17mpg

the last time I put in 8 dollars and it gave me 2 something gallons and right about now It's at the empty line and has only given me 20 miles so far.

I don't know if this is okay or not, I don't know much about cars, but I recently got a new air filter, new spark plugs( gapped at 0.44) and I have added fuel injector cleaner too.

Could my alignment be affecting it cause it's kind of bad right now