As requested, I am going to give some of the details about how I got over 28 MPG on a trip from NC to FLA in a 84 500SEL. We drove about 2000 miles over a 4 day peroid.
Now, to start out, I have heard of 300SDLs geting over 30 mpg so I was thinking maybe I could do it in a gas car. You have to put up with some things (rough ride, poor idle, poor performance at low speed) which I will explain later. Here are the highlights
1. Service with synthitic oil front to rear including P/S, trans,engine,diff.
2. Tires inflated to max cold pressure listed on the tires. (rough ride).
3. Ignition set as high as possible without spark knock using prem fuel.
4. MSD Ignition with plugs gaped at .60 inch.
5. 2.5 inch exhaust front to rear with free flow exhaust. Some of you have heard it and it is not that bad when inside the car.
6. Fuel trim set lean at 2500 RPM. This engine doesn't have a computer.
7. Engine has Euro heads and cams.
8. Now this is the kicker. I advanced the cams 8* at the crank.
9. I would draft big trucks when ever safe.
10. 195*f thurmostate to keep engine hot.
11. 95 % Interstate driving.
12. NO agressive driving. (that was the really hard partof the trip)

What a lot of the above did was make the engine more efficent at 2500-2700 range but hurt it at low rpm. Idle was rough, high speed was powerful, and mid was efficent.
Did I leave the car like this, No Way. I usually get around 20 MPG on a trip but the car runs so smooth I just love it. The above was just to see if it could be done. I proved to myself that you can move the "Sweet Sopt" around for different conditions but you can't have them all without modifications morethan what was done above. Some of these things are still there but some are not.

OK, let it fly, I am ready to defend myself, and answer any questions if anyone has any.
Paul

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84 500 SEL (307,xxx miles)