Why might a 1998 Honda Civic be getting poor gas mileage even after tune-up?

- Bad oxygen sensor: a bad oxygen sensor gives false information about air-fuel ratio to the engine control unit (ECU), leading to poor gas mileage.

- Dirty fuel injectors: dirty fuel injectors can cause the fuel to be sprayed incorrectly, leading to poor combustion.

- Low tire pressure: under-inflated tires increase rolling resistance, forcing the engine to work harder and consume more fuel.

- Faulty thermostat: a faulty thermostat might cause the engine to run at a lower temperature, affecting fuel efficiency.

- Transmission problems: transmission issues, such as slipping or a faulty automatic transmission lockup solenoid, can reduce fuel efficiency.