It all depends how there tuned,most guys wanna smoke the tire with there diesel pick ups so ya tune it for hp or torque or whatever.If ya tune it for mpg it's suppose to give ya better mpg,i trust the guy cause i've purchased thousands of dollars worth of his injectors which are 20% cheaper then ford and Iternational and have a 1yr unlimited mile warranty.I drive a gas van with a k@n and also had a 7.3 diesel with one,now i just had the filters not the whole system.I did not notice the mpg go up however there worth it cause ya don't have to buy filters.I will try the tuner i know i'll get refunded if i don't like it for whatever reason,again gotta get driver and van over there.I have broke 20 mpg with since i replaced the injectors in my 550,000 ford.I drove it the 1st 100,000 and even with the wind at my back empty 19 was the best,it would avg 18 now it's avg 19.Before they did injectors they did a dyno and compression test to let me know if the engine was worth the injector replacement all checked out so away i go to for at least 800,000 I had them due the egr cause it was due and there price was 169 bucks,i had to buy one from ford once 379 bucks.I clean them around 250,000 thousand,instead of having them go out and get hit with the ford price along the there crazy labor.