I have a 2004 chevy silverado 2500 HD 6.0 with 330,000 miles. Runs fantastic, looks like new. Take really good care of it. Recently my gas mileage has plummeted from ...

I have a 2004 chevy silverado 2500 HD 6.0 with 330,000 miles. Runs fantastic, looks like new. Take really good care of it. Recently my gas mileage has plummeted from 11.5 to (8.5) and with gas at $4.00 a gal in calif. Its killing me. Plugs are not bad, breaks are not binding, running 100% synthetic. Tranny flushed every 35,000 .
Only thing I haven't done and thinking might be the culprit is fuel injectors. Never done. Any intelligent advice welcomed. Thanks again

First things I would do is plugs, wires, fuel filter and clean the throttle body especially the idle passages as well as mass air flow sensor. Unless the plugs and wires have been done recently just because they look good does not mean they are performing. I usually do plugs every 1-2 years and wires every 3-4

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O2 sensors ever been changed?

Move??

Plugged catalytic converter will result it horrible MPG. You would get a distinct "plugged" sound to the exhaust and/or rattling as the ceramic insides break up. In severe cases, your exhaust manifolds will also literally glow a dull red-hot as gases are backed-up and have no where to get rid of the heat. Given you have 300K+, this is a distinct possibility.

Do you have a check engine light on and if so what are the stored codes?

So I was curious what you would be spending on fuel assuming you put all those miles on that truck. Here is what I come up with:

330000/14 = 23,571 miles driven annually

23,571/x = 8.5mpg

23,571/8.5 = 2,773 gallons of fuel used annually

2,773*4 = $11,092 annual cost of fuel

Is that correct? If my math is correct that is ridiculous. I may have missed something somewhere in my calculations though.

When as the last time you changed the air filter.