I drive my '96 one mile to work each way. It will get the same bad mileage because the computer sees the engine running more than the odometer sees road miles. I get the car out on the highway and I’m back to the mid 20’s (MPG). Real low city miles will kill anything. Now, regular city miles should get you more. I average 20, but that is with 10% ethanol gas (we got to run this crap in southeastern Wisconsin). Real gas gets about 1-2 mpg more. I’d do like
tedhontz says and check the O2 sensors.