i run a snorkle on alot of my trucks because the fact of the matter is around here its not uncommon for me to be chewing through a hole the likes of which most of u guys would be screwed in. i have run MANY snorkles and nothing ever decreased gasmileage, its the same concept of a long ass cold air intake, doesnt do dick for performance.
the fact of the matter is most the time im in mud/water thats EASILY coming over the cowl of my truck chewing through a hole, if u plan on playing in the deep stuff the a outside the truck snorkle or in the cab snorkle is a must
i ran one on a toyota with a 22r and it killed the mileage, snorkle was from the carb though the fender and up the side of the cab