YOu have not mentioned what your driving is like. If it's city driving only then 20 MPG isn't terribly off. I'd say if you barely took it over 2400 RPMs ever then there would be something wrong. But I bring my car up to 4500 RPM every time I drive it and I get 20 mpg/city. This is with valve adjustment and turbo adjusted to 13 psi in 4th gear.

Will jump to 25 on the highway.

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1985 CA 300D Turbo , 213K mi