Three years ago I owned a 91' Chevy Cavalier. 2.2L TBI 4cylinder with DIS igniton.
My best mileage was driving in a light rain from SE Michigan to SE Illinois(Urbana)
300k on the motor, but had just changed
-the head gasket(felpro)
-the plugs to copper autolites (autozone wires)
-Mobil One oil
-decarboned the piston tops
-the head (so all springs, guides and seats were in spec)
AND tires were 205/75R14 Yokohama Aegis LS4's with 7/32's tread left inflated to 40psi (tho the sidewalls began to ripple)
39.16mpg (average of two tanks to Urbana(I'll never forget that), 35ish mpg on the way back(traffic was moooovin at 85 tho)
...all the while using 86 Sunoco. I'm a low octane believer, least on that engine.
"You have to know the truth, and seek the truth, and the truth will set you free."