How can the wrong fuel filter cause poor gas mileage? Fuel goes in, fuel goes out, simple. Chris hlava wrote; > I received a fuel filter from one of our 20% discount vendors(a Nissan > dealership). I installed it... got terrible mileage for the 1st 2 > tanks(22.5) and only slightly better for the next 2(23.5). Before > I changed > the filter, I was getting anywhere between 26 and 27.5 mpg.