I drive my SD like an old lady and get 25mpg. This thread makes me interested in swapping in a 2.88 diff. My 85 300D California with the 2.88 runs like a completely different car.

Anyhow. When I first bought my 82 SD I had similar mpg. Like 20mpg or so. I looked everywhere for the problem - flat tires, dragging brakes, fuel drips, tuning.

I finally found out my odometer was slipping. I wasn’t clicking off enough miles between fill ups. It was especially bad at highway speeds so taking long trips didn’t really improve mpg which it should have in theory.

Try downloading an odometer app on your phone and take long drive. Mine showed I was reading 20% less miles. This was caused by some worn components in the electromechanical speedo. Took a few years (mostly because I didn’t care once I knew it was just the odometer) to sort out.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles