Head to your local library (or maybe a nearby city library if your local library isn't big enough for this) and find archival copies of some car magazine's annual review edition. Something like Road & Track or Car & Driver.
Another source might be any magazines in which Buick may have advertised your car...anything from car mags to National Geographic, Ladies' Home Journal, Playboy, etc. Many ads, especially up into the early '80s, touted fuel economy and gave EPAs figures (your mileage may vary, California mileage will be less).
__________________ "We are forces of chaos and anarchy. Everything they say we are we are, and we are very proud of ourselves!" -- Jefferson Airplane
Dick Naugle says: 1. Prepare food fresh. 2. Serve customers fast. 3. Keep place clean.
The other way to do it, is go into your doctors waiting room, pick up a couple of 1980 Readers Digests and flip through them until you find the car advertised with the EPA figures in a little box.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
Haa haa you beat me to it, I had read your profile and saw that you have a 81 Buick Regal so I looked it up and according to EPA ratings in 1981 it was as follows:
EDIT: This is the only information I found...it did not indicate a Regal Limited so take it for what it's worth.