What about that water/hydrogen conversion kit that you can build yourself and add to your system? It might be a hoax but I understand that you can build it yourself from common materials and increase your mileage by 40-60%. If it's true, I'd like to try it just for the science of it.

 

check out the hydrogen thread in off topics im making some good progress

 

My 04 Silverado 2500 Crew Cab Longbed 4x4(6.0L,410 gears) was getting @11mpg at time of purchase. I added a fiberglass camper cap (with good aerodynamics,apparently) and my mileage immediatey went to 14mpg. The cap (1400$) paid for itself in about 2 years.