98 subaru forester s

My forester gets terrible in town gas mileage- usually about 11 or 12 mpg. On the highway it gets 27. In town the car should be getting 20ish. I have replaced the air filter, fuel filter, tires, checked tire pressure, replaced the spark plugs, had compression testing done, and a handful of other things Im sure I cant even remember. I have taken it to the mechanic twice only to be told there isn’t anything wrong with the car. I even had a improve your mileage or your money back garantee" which did nothing for the in town mileage and when I took it back the jerk took it for a highway test drive and told me he calculates the mileage to be 52 mpgs- which is just a flat out lie… Im not interested in paying to be told there is nothing wrong with it anymore… Oh, also, no check engine light and no “excessive carbon buildup” in the tailpipe

Perhaps it’s your driving style and conditions?

A couple of years ago, the EPA corrected their earlier, overly optimistic fuel economy ratings, and the result was lower mpg figures than had been posted on the window stickers of cars sold prior to 2008.

For your car, this is what the EPA states:

MPG (city) 18
MPG (highway) 24
MPG (combined) 20

Your belief–that the car should be getting mileage in the “20 ish” category in city driving–is apparently not realistic.

If the EPA estimates that you should be getting ~18 mpg in the city (rather than “20 ish”), and you are not achieving that 18 mpg figure, and if the car has been maintained at least as well as the mfr specifies, then you have to consider what impact your driving style is having on your fuel economy.

However, you should have your thermostat checked, and possibly replaced.
If the thermostat is defective, that alone could have a negative effect on your gas mileage.

Some people when driving around town are always either on the gas or brake and rush up to stop lights. They get lousy mileage. Some people drive less than 5 miles a trip, so their car never gets fully warmed up.They get lousy mileage. Some people warm their cars up before driving. They get lousy mileage.