My former 1996 Dodge Grand Caravan (same model until 2000) would get the same average economy on a 55mph trip as a 70 mph trip. It was modded to 27 MPG. It had the 3.3L v6, 4 speed auto, and 15 inch wheels, also a roof rack. Had to make sure the tire size was correct, and I'm sure some aero mods would have helped too. My 3.8L GC would get terrible mileage over 55 with all other options identical, though it didn't notice a 1 inch tire height difference. Note that these two engines are in the same family, but have drastically different personalities. The 3.3 likes to rev, the 3.8 makes TORQUE. I only miss those vans when I'm at the pump.
