My fuel mileage in my 2005 2500 diesel automatic w/3:73 gears and stock size tires suddenly dropped about 2 mpg last month for no apparent reason.

I am using the same brand of fuel, same station, driving habits the same, tire pressure all good & the same. I have been checking & evaluating for about 1 month now.

Any thoughts would be appreciated.

I experienced the same thing with both of my diesel trucks a few months ago. Like you, I had been getting fuel at the same place for both trucks and mileage was down on both of them. One is a 07 6.7 Dodge and the other is a Mitsubishi Mighty Max that I had converted to an oil burner. Both are well maintained and nothing was wrong with either but the mileage was down about 2 to 3 MPG. I thought to myself the only thing that it could be was was the fuel. Turns out I was right. Started getting it from a different station in town and mileage is back to normal on both. This crap they call diesel isnt worth a dang anymore. It has no shelf life and dont last like it used to. Just like back in the day you used to be able to start an old diesel tractor or truck that had sitting for years with the same fuel and they would run as good as the day they were parked. Not anymore.

While I do agree on the fuel quality today check the engine oil level and make sure it isn't rising, an injector could be leaking into the crankcase.

I been seeing more places with a 10% bio mix in the diesel... I know on my last trip.. It was 21.4 down, 10%blend home 17 back..

yes, B-20 BIO will have less BTU's in it, and less fuel mileage... make sure what you are getting at the pump.... it makes a big difference... some stations here are not labeling there pumps as B20..which is a no-no...

For sure, the easiest to try first would be a different fuel station even if you have to go out of your way to find a high turnover place.

Your air filter should be good but check it anyway and then on to a new fuel filter.

It looks like this could be the problem as the oil level is above the full mark and appears to be climbing on the dipstick. It is now about 1/4 inch below the part number stamped in the dipstick.

This started just after I rescued my neighbor and pulled his travel trailer back to Phoenix.

Is there a diagnostic check to determine if it is an injector leaking into the crankcase?

I will let you know what I find out.

If you are "making oil" you might check out this thread:

http://www.cumminsforum.com/forum/3rd-gen-powertrain/476082-05-5-9l-rise-oil-level.html

a leaky nozzle can dilute the oil, but is usually accompanied with smoke, and possibly running a little rough. You can also have O rings leaking which is where it was mentioned before about removing the valve cover and running the engine checking for fuel coming up around the injector base..... stay CLEAR of the injector wiring while it's running. If you are in the Phoenix area i beleive Southwest Diesel has the equipment to test common rail injectors. If you pull the injectors yourself, be sure to replace the cross tube o rings too.

Oh yeah, you definitely don't want to touch any wiring or anything else for that matter while the engine is running with the cover off. You could lose some fingers from high pressure fuel spray or be electrocuted from the voltage those injectors operate at.

High pressure fuel injected into a vein will kill you too.

Do you have any aftermaket fuel pressure enhancing box on your truck? i.e. Egde EZ, Diablo Power puck, etc...? This aggrivates the problem with internal injector leaks. They are known for pushing out the small check ball on the side of the injector body causing fuel to spray into the crankcase. If you are not afraid of making a mess, you can take the valvecover off with it running and unplug the fuel contol actuator. It will raise the fuel pressure to max and you will be able to find your problem if it has one. It will steam and spray around the body of the problem child injector. Be careful though, and stand back from the engine and do NOT lean over the top of the engine while doing this.