At 70 mph, one gets ~30% poorer mileage than at 55 mph. That was the rational for the national 55 mph interstate speed limit during in the 1970's "gas crisis". The cars then had more air drag so probably a lesser effect for a 1987, but not much. If you want really great mileage, follow a tall semi-truck, but I find that only practical at 65 mph or less. In our 2002 minivan, that increases our instantaneous mileage reading from 24 to 33 mpg (if no cross-wind).