I would agree that the K&N doesn't filter very well. I ran one on a motorcycle for some time and noticed the dust on the inside of the carburetor and on the clean side of the airbox.

For mileage, one thing I have found was to run the tire pressures high. How high? I run mine at the max rated pressure and still get even wear. Actually I would generally wear front tires out on the edges on all of my vehicles before I started bumping them up. I still have excellent control. They respond faster with less side wall flex.

The tires that came on my Merc are rated at 35 psi. They are some sort of bf good rich touring tires. I plan on putting on some tires rated at 51 psi before too much longer.

The down side is that you will have a rougher ride.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.