Highway and speed means everything. Here in Texas - no hills and no turns. Cruising on a pretty day with the AC off at a "reasonable" speed and you can get the most mpg of any car/truck.

and, it was on topic, he had 2 questions, one was about the mileage, the other was about the searching... but to give you a topical answer on my experiance, NO IDEA... neither of my cars have working odometers...

So what,. I reckon that the huge drop in milage with the AC on is natural wiht these or is something amiss with my 95? You can certainly hear the compressor on, and in fact, its is THE soundtrack from WWII movies for any military truckmoving about, a whirring "geary" noise. ( and I kinda dig it) System checks OK, cools like stink, but had a high pressure issue when I was romping the heck out of the engine. ( AC would turn off )

Running AC off though, is major milage zone.


BTW, as a numbers guy by trade, I have to say, I wont quite milage. Yet. Why? Too few datapoints, too many exogenous variables. Fillup repeatability, weather, etc. And many of us are relatively new to these cars., so still have few datapoints. But I can see the range differences per tankful pretty clearly. Nonetheless, its easy to get an "outlyer" with mpg calculations/measurements, so much of the exaggerated milage is likely numerical outlyers.

Or out right liars. ;)

'83 SD, I get 29 mpg mostly highway (no A/C windows down, sunroof open), I check odo against the Interstate markers, pretty close. Dino only, WINGAS, bio. etc. I understand does not have as much energy in a gallon, meaning you will not get the mpg you will with dino. Same thing with blended gas (ethanol) in cars, the energy is not as much as gasoline = lower mileage. I drive 70 (65 Interstate) miles a day going to and from work. The valves have been done before I bought it, Nov. '06, I have replaced the fuel cap gasket, some would slosh out during left turns, and now I'm, at least on the way home, driving at around 2500 rpm. (55 to 60 mph) According to MB, that is where THIS engine makes maximum torque, which will give the best possible mileage.

1987 300SDL, 135K

Nashville to Little Rock round trip average 28.05 MPG. Almost all highway on I-40 at 70-75 MPH. Some construction and traffic around Memphis, some town driving in LR. A/C on. A few big hill climbs on the TN side.

When all city…….22 to 24MPG, mostly light-foot with A/C max.

I think this is great for a big, safe, family cruiser car. I could almost double that in a TDI Jetta, but I would have to drive a Jetta to do it……….. Driving a Jetta would have saved me around $35.00 on the ~700 mile trip. Not worth it to me. I am not able to calculate the environmental or foreign oil dependency impact of my decision against the Jetta ;)

Just a note, you can get better mileage, it appears, by using B20 (20% biodiesel mix).
That way you're getting the better lubrication of the bio while not sacrificing the higher energy content of the Diesel.
It's only a 1-2 MPG improvement, but it is an improvement.

B100 gives 2-4 MPG worse than D2.

My non turbo 1979 300CD, gets a consistent 30 to 32 miles per gallon. On the other hand, my 81 300CD turbo car, has never done any better than 25-27 miles per gallon.. I don't know a lot about the diesels yet, I'm still learning. It's weird, but I guess some do a lot better than others.

Thanks all, this has been a fun thread to read. Keep up the answers, it is interesting to see the varied responses.

On a seperate note, I thought that the 86 and later cars did not need a valve adjustment as they had hyraulic adjusters?

Some responses have mentioned adjusting the valves. This is not necessary with your engine, which has self-adjusting valves. Your engine, the OM603, only appears in the 86-87 300 SDL and the 87 300D. A bigger version is in the 90-91 350 SD/SDL. So really, you only need to compare your mileage to other 86-87 SDLs. Even so, most seem to report considerably better mileage.

You say the odometer is OK - that's good, but I'd still look at a longer drive of a known distance. Odometers have been known to slip sporadically. You also need to make sure the tire size you have on the car are roughly equal to what originally came on the car, otherwise you will have to adjust what the odometer is telling you. Of course, make sure your tire pressures are good.

You say you're new to diesels - well, they respond well to robust treatment. You may due well to allow the engine to get good and hot, then put a full load on it by driving up a long steep hill. This will allow your vehicle to burn out any carbon build up that may be in the engine. You also may want to look into running diesel purge through the car. Many report better mileage and smoother idle after this treatment.

Speed is important with these cars. I think the torque peak in top gear with your car is at around 65 mph. Generally speaking that is the point at which you will obtain your best mpg; going faster or slower in the same gear will result in reduced efficiency. Somewhat surprisingly, most report running the A/C doesn't result in a big change in mpg.

Good luck.

If you have a 603 engine? I don't know what the SDL from 86 has but I think most SDL's have a 603 in them. If it has the 603 then yes... no valve adjustment because hydraulic lifters. Also, many people here are not driving the same car/engine as you and are commenting on their MPG's. Not that that's bad but obviously they will get different fuel economy than you will.

There's more than the air cleaner also to check. Things like EGR, Turbo shaft spinning freely, etc. Probably a much longer list than I could give if I sat and thought about it for even 10 minutes here. But that's what I'd do. Look around and make sure everything is working.

Torque peak has nothing to do with mileage, little to do with anything really. What will tell you the most efficient RPM would be a BSFE curve, and to take full advantage of that you would also need to be able to re-gear.

You will find that your car will get better mileage the slower and smoother you drive, provided you're in top gear. The largest consumer of power while driving is wind resistance/drag, the increase of which is logrithmic with regard to speed increase.

Significant contributors to fuel mileage will be first speed, then how clean you're running (windows/sunroof open increases drag, so would a bike rack etc.). Tires are important, good round tires as well as proper inflation, and although ride and handling could suffer the harder the better up to the sidewall max. inflation if mileage is the only goal. Wheel alignment is important, the more the wheels are creeping sideways to go straight the more fuel burned, brakes dragging will cause same, and yes, the A/C will make some difference.

In the winter as one has mentioned here, the winter blend fuel has less energy content, so less miles/gallon, engine tune will matter, even good engine oil will make some difference, as will whether your fan clutch is completely disengaged.

The last thing I'll mention is fuel leaks. It is surprising how much difference can be made in fuel mileage if you have a couple of injector lines seeping a little, 60ish miles of leaking takes about an hour, how much will a couple of leaking injectors leak in an hour?

Brian gets around 30 with his SDL on summer fuel. After seeing how he does it I beleive it. Here is how:

Shut down the AC.
Drive on nice flat open roads for 400 miles, at 65-70.

You will get about 30mpg.

Drive around town you will get low 20's.



Lots of diesel drivers lie about there mileage. I have had people swear up and down that there 300D got 40mpg and there 240D never less then 35mpg.:rolleyes:

I don't pretend to understand everything, yet power and torque are forces that have a great deal to do with engines. Check this out. http://craig.backfire.ca/pages/autos/horsepower

In my 83 300SD 216k, I got 28mpg+ the last time I checked. This was on petro, mostly highway with a little city here and there. The odometer works, at least it indicates the same distance as google maps tells me it will be to get places.

The valves were adjusted by a previous owner 6 or 7 years ago but in general the car seems to have been neglected/abused since then... my A/C doesn't work, so none of that... other than that, I don't know what makes it relatively good. Luck? My old 300TD Turbo got in the lower 20s, but I have since realized after reading some more that it probably needed a valve adjustment.

I'd be interested to see what I get on this tank, which is about a B33 blend.

The 87 300D I had got between 27 and 33 mpgs. Pure highway driving got me the 33 and mixed driving got the 27. This was a car with over 250K on it.