I use K&N filters in every vehicle I buy. Best I can tell, it is good for about 1/2 mile to the gallon inprovement on my '00 GC and '98 Cherokee. However my Dads 1 ton Dodge Diesel it makes about 2mpg difference. The main reason I use K&N is for the long term benefit of not buying another filter and only having to clean it every 50K miles. A low restriction exhaust system can give you 1-2 mpg on the highway. If your only purpose is mileage, the amount of $ you will spend getting 1or 2 mpg improvment will take a long time to get back. Usually getting dual exhaust is for the sound (it's a guy thing) and more power (still a guy thing), what guy cares about mileage if his truck has a good growl when you stomp the loud pedal?