Poor Fuel Economy

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Spent the last month in Las Vegas and as usual was browsing the For Sale ads. Came across an '84 300D Turbo which, to make a long story short, I wound up buying. Reason it was being sold was this 79 year old retired oral surgeon and his wife wanted to buy an SUV to drive when they go skiing!!! I LOVE THAT ATTITUDE!

Drove it to Houston and dropped it off to my son. He was going to buy his mother's 300D Turbo, but I didn't really want to make my 500SL an everyday driver. Thus, the search for another.

The car is white, no rust, original paint still in superb condition, a few small 'door dings', excellent interior, 192,000 miles and worth every bit of the $3,500.00 paid for it. You can't hear the engine in it while driving 90MPH out in West Texas. Literally runs like a champ!!!

Only problem, which probably really isn't a problem, is the mileage I got--22 MPG. Ran a couple bottles of STP diesel treatment through it along the way, but that didn't seem to help any. My 300D (5-spd) gets about 28 MPG and the 300 Turbo gets about 25MPG. This one seemed to run about 400 RPM higher at 75 MPH than the wife's, but it doesn't seem this should be enough to cause the decrease in mileage. I suppose the injectors might need to be rebuilt, but wonder if there are some other things we might try before going to that expense.

Any help will be appreciated.

Also, for those living in the Houston area, my son has an '80 300D that he's looking to get rid of. It's has a little rust, and a poorly shifting transmission that may be vacuum related, but is very driveable. He's had to put a couple glow plug relays in it and can't seem to find out what causes them to keep going bad. TX Bill was going to take a look at it, but I believe he got sidetracked with some of his other projects.

Sorta apologize for the long post, but most everyone likes to brag when they come across a good deal. I'm no exception!

Cheers!