My dad has an 03 6.0 and would like to get better fuel mileage can you guys give me an idea of what to change to help up his fuel mileage?

 

Use a lighter shoe on the right foot.....

Seriously though, steady rpms, smooth acceleration and lower top speeds will all help, a good tune from a reputable tuner will help a little bit also....even a well-tuned 6.0 will never see better than 20mpg hwy (15-17mpg hwy is average)

 

Best power and mileage will occur when the spark table is configured for best torque. To do this, you will need to dyno tune the truck, as there is no other way to figure out what timing advance gives you the peak torque output.

Beyond that, having an accurately tuned VE table helps. If you want to really go all out, you can upgrade the PCM to a custom operating system and command a leaner AFR for light acceleration and cruise ranges. I'm in the process of doing this with my SS right now and am already seeing promising results.