Let's do some math.
The drive to my job is 13.3 miles. I drive there and back once per day. It's 98% highway. (~27 miles.)
I work five days per week. (135 mi/wk)
I usually have to put gas in once a week. I put in $20. [$20/(~$3.30/gal) = 6gal/wk]
If I'm correct, then that's (135 mi/wk) / (6 gal/wk) = 22.5 mi/gal. Is this about what's expected of my car? It's a D15B7 engine.

If not, maybe this has something to do with it.

I've got a CEL or two. I've reset it to make sure it's legit. It's OBD1 so it flashes 4 long and 3 short, long pause, then 1 short. Decoded, that's code 1 (Oxygen Sensor) and code 43 (Fuel Supply System). Via the Service Manual, troubleshooting code 1 either leads to a bad Oxygen Sensor or leads to checking for code 43.

Don't know what else you'd need to know about my car or how it runs to help the diagnosis, but feel free to ask. I'll follow the troubleshooting guide in the Service Manual, but I'd also like to have input from real people.

 

Your car's eating so much gas. I get 35 mpg, others do better. you are running rich. Go fix the trouble codes first.

 

Replace the O2 sensor. Code 43 will probably go away then. Also check the spark timing.

You need more accurate records to know the exact mileage (Is it really true you only drive to work, no side trips or extra trips to go shopping or visit friends?). It's simple, just every time you buy gas, write down the odometer miles and the number of gallons you put in. After a while of doing that you can make a grand total of miles versus a grand total of gallons and divide it out.

 

^ +1. Op, you are not calculating gas mileage correctly. You are probably getting around 25-27. I get 25mpg and my car feels like its going to blow up lol. Replace your o2 sensor(very easy), don't go to a mechanic or it'll cost you a lot. And if its still throwing codes, replace your fuel filter.