I agree with Forced, the Germans used to know how to build things. leave it original, and tune it up properly, and remove the epa b.s.

So basically what I am taking from this thread is that with the way these cars were originally designed; if you take care of the maintenance and don't hammer the accelerator pedal every time you take off; this is how your best mileage will be obtained.

My '83 300SD will average 32 on the freeway with cruise set to 55...

I have read some misleading posts below. About Mercedes airbox being as good as a cold air system, and the exhaust system being adequate to get the best fuel mileage out of these cars. That's bunk! Have you ever known a car manufacture to do anything more than just meet EPA standards and the least amount of cost to get by. I have a 1986 190D. I removed the original Mercedes filter closed up the airbox and install a 90° tube with a K&N performance air filter, on the original inlet of the air cleaner box. And then remove the three muffler exhaust system and went to a two and three-quarter inch pipe from the front to the back with a high flow muffler and a turned down tip. The name of the game is airflow maximum on airflow in and out. One other thing I do, I mixed to a half ounces of two cycle outboard motor oil to every 10 gallons of diesel fuel, to counter reacts ultralow sulfur diesel fuel that is now sold. This alone can save you as much as 1.5 miles per gallon. And you'll be surprised how much quieter your injectors are.

Originally I was getting 37 miles to the gallon. Now at 65 mph I'm getting between 45 and 46 miles to the gallon. At 60 miles an hour I'm good day and touching 50 miles to the gallon. I'm a class VIII truck driver I have been practicing fuel mileage since the early 80s. In the early 80s averaging 4 miles per gallon. In year 2012 I averaged 8.6 miles to the gallon the 2004 Volvo 780 550 hp Cummings.

Brian, I had my first '64 190d almost 50 years ago and several others since. I now drive an '87 300D Turbo. It's in daily use with 248,000 miles on it. (It's FOR SALE too, $5,000 or my 1998 E320, 166,000 miles, same price, your choice.) I too have been a fuel conservation freak for many years. It's the total cost it really comes down to and that is why I think an old MB Diesel is as frugal an operation as you can get. About cleaning up airflow. This does not apply when you have a Turbo. You get 16# boost and that is as much as the engine can handle. All you have to do next is make sure that there is no problem with fuel delivery and that the air filter is not clogged. My check for that is to step on the 'gas' and watch the acceleration under a load, as when entering an interstate. After you are in third gear, if the RPM marches right up to 4,000, that's good enough. Don't look for trouble. The factory advertised fuel consumption, if I remember correctly, was 29 mpg at 120 km/hr and 3/4 load. Top speed of 125 mph. Slowing down is what increases fuel economy! I have records that show this car will get 34 mpg in suburban driving between 40 and 50 mph. Keep it below 60 mph and you can get 32 mpg. On the interstate with only myself in the car and driving no faster than 80 mph I can get between 29 and 30 mpg, depending on how hard the A/C works and on whether I have head or tailwinds. Diesels require special engine oil, I don't think there is a point in substituting synthetic oil. It's more important to just change the oil regularly. Keeping the oil clean and not running low on it is what matters. My car burns less than a quart in 5,000 miles. I think that this is great, keeps everything lubricated on the way out. :-)

Did someone insult your intelligence. I've been playing with fuel mileage and fuel mileage results probably longer believe in living. I know what works and what doesn't work. I have seen the results in my automobile. If you think that original air filter comes close to flowing as much As what I replaced it with, just keep thinking that. It's obvious you are close minded and boneheaded. Oh yes and I'm sorry my computer spelled Cummins wrong, next time I'll double check my computer spelling to make sure it spells everything as I transcribed it, sorry. Just by that comment alone I can see you been insulted by being proven wrong!

There is a test the shows that the K&N Air Filters do not filter well.

I put the below info in My Notes from a Member of another Forum who was a Truck Driver:

"I have done extensive record keeping about what improves the mileage on my N14 Cummins, as I am a long-haul truck driver. During the $4.50 plus days, I was going crazy trying everything to increase my mileage above 5.5 mpg. The only thing that consistently worked, was to use an upper cylinder lubricant in the correct amounts. I used a gallon of Lucas, or 1/2 gallon of 2-stroke oil for approximately 300 gals. They were the only products that consistently raised my fuel mileage and quieted the engine down. However, since 2-stroke oil doesn't have an injector cleaner, I had to put that in, either as a dedicated cleaner, or switch to the actual Lucas (or USA Lube) at least once every six fillups. ATF made no difference on my fuel mileage or quieting down the engine, but worked as an injector cleaner, in the right amounts. Unfortunately, the ultra low sulfur diesel does not perform as well on our older MBZ or heavy-duty truck engines, necessitating a regular dose of upper cylinder lubricant. However, I have noticed that Bio-diesel does quiet things down, if at least B11, but didn't make a difference in mileage, and maybe a bit less power."

The above is what the Guy Posted. However, I don't see how anyting dumped into the Tank will act as a Upper Cylinder Lubricant. That is something that could work with Gasoline Engines where the Fuel and Air Charge are inside of the Cylinder for a while.
In the case of a Diesl that mix He was speaking of would be burning as it injected into the Engine.