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cleenspeed, technically any modification (including re-programming) that would result in increased emissions is considered illegal by the EPA. That being said, I have never heard of any individual being prosecuted for modifying their own vehicle.
Most people don't realize this, but for a modern diesel engine the EPA regs (for nitrogen oxides and particulate matter) are THE single biggest thing that hurts fuel economy. I work in diesel R&D, so I could go on and on about this. For a truck like mine there are 2 big things: the EGR system and the DPF. The DPF is the bigger of ther two. You lose mpg's because of the huge exhaust restriction (several times what it would be otherwise), because of the fuel used to regenerate the filter, and because it prohibits good driving styles (you have to be in lower gears in order to keep your rpm's up, and you can't idle much because the DPF gets cold and will plug up).
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