I know this will vary a lot, but just looking for a general idea and experience on how much 1 or 2 jet sizes in the primaries will affect your gas mileage. Obviously a bigger jet is gonna use more fuel, but is a couple sizes enough to notice a big difference in mileage on the highway. Reason I'm asking is I've been playing with the 600 on my truck, and went from 66 to 67 in the primaries. My mileage has since gone to complete crap, and I can't believe that one size made that much difference. And this is driving the same routes to and from work everyday. I changed to 65's this afternoon so we'll see what happens. Thanks.