[Richard]

I had a similar but not as dramatic increase in mileage when I adjusted the valves on my engine. I saw about a 0.5 mpg increase.

Granted I had to adjust the valves and the injectors when I took the head off to replace the cylinder liner.

I know that DD doesn't really call for an adjustment at 100k miles but in hindsight I would recommend it based on my own results. It could be that the injector adjustment was just as beneficial as the valve adjustment.

The valve adjustment is very easy if you read the DD procedure. The injector adjustment requires a specialty gauge that is available on line. Take care to read the injector specs on the side of the valve cover to order the correct one.