I do appreciate your input.

The numbers I listed in the posting were an estimate, either way the estimate using the fuel guage over the past few days shouldn't be off by 40%.

I do track mileage on my 300TD over an extended period tank by tank.

The TE holds 19 gallons of fuel including the reserve. On Thursday evening I filled it up with fuel, it has 210 miles on the trip with the reserve light on. All interstate miles at 70-75 mph

210/(19.0-2.4) = 12.6 mi/gal.

It's been like this for a few weeks now.

I drove the car to Cincinnati and back about a month ago (approx 300 miles on a tank of fuel driving at higher speeds (80-85). I've lost approx 90 miles to the tank even driving at a slower speed (70 mph).

No offense, but I don't need to track it for several tankfuls to see that something is up with the fuel consumption of the car. I am going to start now, tracking the fuel consumption, but have seen no reason to keep the record until now.

You guys have a lot more experience with this model than I do, but it seems obvious that something has changed. It's a symptom I'm sure. One individual suggested a bad O2 sensor or rich mixture.

Does anyone have any other ideas? Would the O2 sensor failure result in an immediate change or would it be noticed over time?
How would the fuel/air mixture get changed without someone changing it?

Just ideas, I am open to all suggestions.

I will begin tracking the fuel consumption on this car more closely now as it is an issue.

Is it unreasonable to expect 20+ MPG from a TE on the interstate?

It was doing better before.