I just turned the air cleaner lid up side down on my 1984 454----------biggest change I saw between my truck was I got better milage with single rear wheels than my buddy did with idenical truck except for dual wheels---they were 4 numbers apart in vin as we ordered them at same time--I had bigger Chapperal gooseneck triler, he had Wells Cargo bumper hitch, both hauling Pro stockers, same speed on the road ( aprox 70-75 on interstate, 4.11 gears)I'd get 8mpg he got 6mph---------