8.5 mpg is horrid, but insure you are measuring it accurately. Use gas station fuel pump readings (fill-up to fill-up) and mileage markers (or GPS), not the tics on the dash fuel gage and odometer. I carefully recorded mileage in my 1969 Dart w/ slant-six in the 1980-90's and consistently got 22 mpg highway, back when the national speed limit was 55 mph and before 10% ethanol. At 75 mph, expect 30% poorer. If truly that bad, it may be running rich to put raw gas on the cylinder walls, which is why engines used to wear out faster in the day.