The results are in - Just filled up last night.

I had 370 miles of stop and go, warm the truck up in the morning, 20 mile at a time errand running. Then I had about 120 miles of highway driving (65mph on my 31 inch tires). With a speedometer correction of approx 6% (arrived at by a GPS test last January) I am estimating I had 515 miles on the tank of fuel. Took 19.86 gallons to fill up for a mpg value that is a gnat's leg short 26 mph

Before the K&N air filter installation, 25 mpg is the best I could have hoped for in these circumstances. My conclusion is the K&N added pretty close to 1 mpg.

NCDiesel