Drop-in K&N Air Filter WHY?  The manufacturer's web site says that the drop-in replacement air filter will "increase horsepower and acceleration."  Many truck owners purchase this filter looking for easy increases in power and/or economy, but I've seen very little hard evidence to prove its effectiveness.  I decided make my truck the guinea pig and put the K&N claims to the test.  I could have purchased the filter at any number of dealers, but needed to buy a few maintenance items from my favorite auto parts site, Courtesy Nissan, so I placed the order on their site.  It is clean, easy to navigate, and it is easy to place a secure order via credit card.  Their price was $44.95 plus shipping.  I placed an order for the guards and some assorted maintenance items. SERVICE.  Fantastic.  Immediately after ordering I received a confirmation email.  Within 24 hours I received an email telling me that the package had shipped via UPS.  As I am located in the same state, the goods arrived 15 hours later.  Incredible!  This is the type of service that I have come to expect from Courtesy, and it is the standard for internet sales/service that should be the goal of every vendor. INSTALLATION.  This job could be accomplished by most any owner and required no  tools.  The filter dropped into place in the stock air box in less than a minute. RESULTS.  If the point of this purchase is to increase performance the results are easy to measure; a trip to the dyno showed that there was virtually no difference in performance between the stock filter and the K&N unit.  That doesn't really surprise me; the airbox (and air filter) on this truck is the same size as that used on the V-8 Titan and Armada trucks, so I would suspect that it flows all the air that the 4.0 V-6 needs.  Early indications are that fuel economy is little affected, but I'll keep a log and measure the results as I add miles to the truck.  For dyno charts of the filter test, click here. WORTH THE MONEY? Maybe--if you want to buy a lifetime filter.  If you are looking for a little more horsepower you might want to spend your cash elsewhere.