I am looking to see what some of you have done to improve you gas mileage on the 6.7. Any suggestions to improve with aftermarket parts (cold air intake, etc.) that will not void the warranty. What fuel additives are you adding (if any)? Which fuel additives do you belive work the best? I add fuel additive everyother tank and average unloaded highway around 19mpg. With out the additive I average around 14-17 mpg. This is a Tow truck so with the added weight of the wrecker unit, thats great gas mileage. Suggestions?