I will probably get a lot of disagreement about this but here goes. Engins are like people, there are all different. Every engine has a "sweet spot" where they are "happy". They usually make the most power at this "sweet spot" and therefore get the best mileage at this spot. People are all different and drive differently. They drive by the seat of the pants most of the time and may drive different cars differently to get the same feel. In Brians case, he drives in this "sweet spot" more of the time in the 116 then he does in the 123. Finding this sweet spot is not hard. It takes many trips over the same distance (say to work and back) at different average speeds for a tank of fuel. I'll give you an short example. In my 84 500SEL I made two trips to southern Flordia from North carolina. 1500 miles roundtrip each. On the first trip I usually ran between 75 and 90+ and averaged about 25 MPG for the trip. The second trip I had two of my Aunts with me (one 84 and the other 94) so I drove the speed limit of 65 to 70 or slower at times and only got 18 MPG average. Nothing changed with the car (maybe a oil change with the same oil I always use). There was 4 people in the car both times, about the same outside temp (hot). That 500sel loves to go. It feels happy at about 90, but is a chore to keep it at 60 if not using the cruise. I believe 90 MPH is its sweet spot. On the other hand my 91 300D 2.5 likes about 65 and it seems happy there.
However, a different 500SEL or 300d may be completely different from mine. They are all different.
By the way, I have been driving and working on car for 50 years or so and have seen this many times.
There are so many things that effect mileage that no one can explain why 2 different cars will get different MPG.

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84 500 SEL (307,xxx miles)

My 300sd went 383 miles and burned 10 gallons of diesel, which obviously translated to about 38 mpg. That was from Winnetka, IL to Des Moines, IA, at an average speed of 50-55 mph (don't ask why so slow) without any stops and with the A/C on occasionally. That was last August. Right now the car averages 25-26 mpg, and thats mostly city driving.

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73 450SL AMG "Millennium Falcon"
88 Volvo 740GLE B230F/M47
84 300SD Turbo (Sold, looking for new one)
86 300E (FOR SALE)
78 450SL (totalled by girl on drugs in SUV)
87 Volvo 740GLE wagon (totalled by Ford Expedition)

there is simply no way that a 500sel will get 25mpg running 75 to 90. you must have omitted a fill up in your calculations. i own a 500sec. i have kept meticulous records over at least 100 tanks, the car runs perfectly, and the best ever is 19mpg running 65. there is no way that a sedan running thesame engine is going to do much better. no spot is that sweet.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Well in my case you are talking two cars with the same engine (OM617 turbo)...the heavier older one with lower (higher ratio) gears trumping the newer lighter car with higher (lower ratio) gears in milage...

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I agree with this. It is impossible to get 25 mpg at 85 mph when you only get 18mpg at 65 mph.

A "sweet spot" is nothing more than an anomaly where you don't have the data to support the results.

The results of Pmckechnie violate all the principles of air friction over the body on the order of 50% greater fuel efficiency at higher speeds. It's not possible.

I knew I would get some disagreements. First, Let me explain a few things about my 500SEL. It is a US spec car but has euro heads and cams which means it has a little higher compression ratio, and the cams were designed for high speed driving. I have seen the speedometer go past 140MPH and I checkened out and let off. The car also has a 420 fuel dist with a modified 420 computer system for fuel management. The 0 to 60 time is more than a friends 87 420 so takeoff is a little slower than some other 500s but when you let it go, it will fly. So you can believe it or not, this car gets better mileage at 85 then it does at 65. Now I must admit in town it gets 10 to 12 MPG where as my 300D gets about 25 in town and only 30 at 65. The 500 has become the the car for trips and the 300D is the daily driver. Boneheaddoctor's two cars back up my statements about differnt engins have different sweet spots. The 116's sweet spot is at a higher RPM then his 123 thus it is giving better mileage. Agreed, there is no data to explain this sweet spot but if someone could figure it out and give all cars the same sweet spot then all cars could give the same mileage.

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84 500 SEL (307,xxx miles)

for running 65 is correct. the other can't be right... sorry. it violates all rules of physics which unlike politics can be proven.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

i get 26 weather i am deep in the city or speeding down the interstate


1983 300 sd

T walgamuth,
I usually come to this site for help but you have caused me problems. I have always heard that rules are made to be broken but I don't like to brake rules. I have been working on this car for 3 or 4 years, learning about how it works, making improvments, and all to no avail. You have now told me that I am breaking rules, so now I have to face my dear 500 and tell it it has to stop what it has been doing so well because you say so.
I am sorry I have been breaking your rules.
I think I will leave it up to my dear 500 and let it deside for itself. The more I think about it, I didn't brake the rules, the 500 is the one at fault, so I will just let it do what it wants.

If anyone else would like to know some of my secrets, let me know and maybe I can help you improve your mileage (gas cars only). As for t walgamuth, it can't be done because it brakes rules so he wouldn't be interested.

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84 500 SEL (307,xxx miles)

my problem is my brakes dragging. maybe that is why my car gets less mileage when i drive faster. as i speed up i must subconsciously put the brakes on.... otherwise as i go faster i would surely get better mileage.

oh well i guess that is just the brakes.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

sorry to be blunt. i dont doubt your sincerety or your honesty. i just doubt your scientific method. you may have stumbled onto the biggest mileage advance ever found.

i just think that the odds are a lot greater that you have made a miscalculation somewhere.

if i am wrong and you are right, you can be rich and i will be just wrong.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

a gas engine needs a fuel mixture to air ratio of between 13.0 and 14.5 ( i may have this a little wrong but this is close). so if a given displacement engine (5 liters) turns one complete cycle (2 rpm) it will intake and burn 5 liters of mix. now if you go below the minimum you burn a piston, over, other bad things.
now unless you have managed to modify your engine to shut down cyliners like the new hemi, then to get the kind of mileage you are talking of you would need a rear end ratio of maybe 1.68 or a tranny with an overdrive ratio of say .35. now in your list of modifications i dont see either modification.
there are prob engineers that use this web site that can do a calculation of how much horsepower is needed to cruise at 85 mph and work backwards from there to see how much fuel is needed to push your car through the air at that speed, but i cant.
i got the above fuel ratios from smokey yunick who knew a thing or two about engines.
the only other explanation is a odo that is wacky, or miscalculation of the amount of fuel used, or a friend who snuck into your yard and added fuel in the night.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Well, this is not exactly true. It may use five litres in two revolutions, but the absolute pressure is normally well below atmospheric pressure (14.5 psi). So, the actual amount of air and fuel used in two revolutions is significantly less than a simple calculation would provide, unless the engine is not throttled.

I'm in agreement with everything else.

I was going to drop this discussion but I think I will try one more time. First there has been many modifications not mentioned. First a few facts. MPG is determined more by efficency then most other factors. Think about an 18 wheeler grossing 80,000 pounds and traveling at 70 MPH and is as areodinamic as a brick. It will get 10 MPG. The big diesels are much more efficent than most any car motor made. Now I have a 4000 lb (?) car that is much more rounded and can slip thourgh the much better. Why would you think I can't get 25 MPG. I'm sorry you can't.
Now I will tell about these modifications but can not be held responsible if others try it and something bad happens. Some can be called dangerous.
1 Cam timing has been advanced 10* with offset keys.
2. The euro distributor has been recurved to get more total advance (48*)
3. Installed a mixture injector in the intake that is controled by an O2 sensor and a homemade electronic device (thats all I can say about it right now) that samples and corrects the mixture at a much higher rate than most fuel computers. Now the fuel dist is set lean and the mixture injector does the fine tuning.
4. A good hiflow exhaust system.

Now I must admit, it probably doesen't idle as smooth as most 500s and I know I lost some low end torque. Low speed mileage is not very good but the car is still quiet, smooth, very good performance compaired to other brands of cars with the same weight. During the tuning process I had an exhaust temp guage that told a lot about the mixture in the cylinders. Did you know that you can run much leaner when going down hill then most cars run. 14.7 to 1 is a good average to run but that can be modified quite a bit depending on conditions.
The reason this 500 gets better mileage at 85 then it does at 65 is because the cam timing has moved the max efficence up to a higher RPM. In about 1 hour, I can remove the cam keys, retard the timing, and change the fuel maping program and return to a screamer off the line, still run 130MPH+ and get about 21 MPG at 65 and 17 at 85. In other words I can change it's sweet spot. I am working on varable cam timing as used on the newer V8, a direct ignition with programable timing, etc.
I know, I could just buy a newer car, but I enjoy doing this kind of things.

End of Story.

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84 500 SEL (307,xxx miles)

Let's look at the fuel consumption in gallons per hour. It's much more enlightening than MPG.

In the case of driving at 65 mph and getting 18 mpg, the fuel consumption is 3.61 g/hr.

In the case of driving at 85 mph and getting 25 mpg, the fuel consumption is 3.4 g/hr.

Now, it is definitely possible that the engine is more efficient at the higher rpm's that occur at 85 mph. There is no disagreement here.

But, the speed of 85 mph requires 2.2X the horsepower that is required at 65 mph.

So, is it possible that the engine can be so much more efficient at 85 mph that it can put out 2.2X the horsepower while simultaneously consuming 6% less fuel per hour?

I hate to say anything is impossible, but there are no plausible reasons for such an increase in efficiency. The only possibility is that the engine is running excessively rich at lower speeds and this condition is corrected at higher speeds.