I don't know how much modifications you have all done to your cars, but my 1990 is mildly modified from stock and gets about 21 mpg in my daily commute. That's about 60% highway, 40% city. I check the mileage at each fillup with a quick mental calculation. (Just out to 1 decimal place, to keep my math skills sharp.)
I think some of these mileage numbers sound awful. Might be worth looking into.
I should also say that my mileage seemed to drop 1 or 2 mpg when the 10% ethanol gas came out. Yech.
1990 SC auto ... red outside, gray cloth inside ... ~205,000 miles (and climbing) ... driven daily!