looking for you guys opinions on diesel tunning. my dad has an 02 duramax all stock but has k&n air filter and muffler cut off. hes all about getting the best mileage and im trying to get him something for a christmas present cause hes too tight to get it himself lol. but im wondering if a different exhaust set up would help and whats the best programmer i can get for better mpgs? or would the nelson tune be better? im open to all options. thanks!