How do I improve gas mileage on a 2006 Chevy express cutaway van. (15 ft box on truck) does not have the box that goes over the cab and is
just a cube. it has a gas powered 6 liter that gets a whopping 10 mpg empty or full. any chips or special plugs or anything that could help? I was told by the dealer when I bought it that it should get 14-16 on highway. was it just a bold faced lie or is there a fix here?