18 mpg is pretty good already for such a big sled. It had a 400 inch V8 and a Turbo Hydramatic 400 transmission and weighed about 4000 lbs.
My Caprice was in the same weight range. Had a 275-CID eight with the 4L60-E four-speed automatic; was able to get over 26 mpg on the highway, which wasn’t too bad.