Tom, let me tell you a little about the engine and maybe you can shed some light on this subject. The bottom end is the original 84 500 that came in the car. The heads, cams intake and fuel system came from a engine that I was told came from a car out of Mexico. Who knows for sure where the car came from. It is a low compression engine I believe because you can not make it spark knock with timing set anywhere from TDC to 15 deg BTDC on regular gas. I did convert to an ICV system as the thing that is temperature operated for fast idle when cold never worked. The fuel system does not have a frequency valve. I switched it to a system from a 380 using the original fuel computer but never got it to run right so I put the 500 fuel distributor back on with out the frequency valve. It runs great. Could this combination explain the higher mileage at higher speed?
This car was headed to the scrap yard about 12 years ago when I rescued it and made it into a wonderful car again. Best $500 I ever spent.

Paul

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84 500 SEL (307,xxx miles)