On a long downhill we drove from southern Denver 365 miles into Kansas. Here's the mileage for driving mostly I-70 at 70-75 mph on 85-octane. I don't remember if any ethanol was in there but the mileage is so good I doubt it. 2004 Honda Civic: 8.4 gallons => 43 miles per gallon 1997 Chevrolet 1500: 18.8 gallons => 19.4 miles per gallon IIRC, the EPA estimated highway mpg is 38 for the Honda and 18 for the truck.