'04 chevy express 1 ton van 6.0 v-8 10-12 mpg makes no difference a/c on or off, driving 65-70 or 85-90. 65-70 now that we have gps. i was suprised that the mpg did not go up once i had to slow down on the highway. i have an extension, 6', 8' ladders and a handtruck on the roof. i got 60 miles more per tank when i had everything left on a job except the extension ladder. i get about 300mi/tank