Any one want to comment on this. It is generally stated that for every 5 mph over 55 will reduce your MPG by 5%. I just got home from the central Florida GTG (Thanks TheDon) in our 500SEL. On one segment of the trip home I had a moving average of 72 mph and got 19.9 mpg. If I loose 5% for each 5 mph then I should be able to get 17% better at 55, or 19.9 + 17% or 23.3 mpg. Well after 8 years of monitoring the mileage on this car, it ain't gonna happen. Well I did get 23 mpg one trip to Florida about 6 years ago. I was running with a pack of 18 wheelers early one morning. I wasn't drafting or anything like that, but just staying in the pack. Speed ranged from 80 to 100 for over 200 miles.
Now why does this car get better mileage at higher speed? I just calculated the mpg of a 126 going 140mph on the autobahn would give it a MPG of about 1, or 26 miles per tank at a 5%decrease per 5 mph. When they finish the bridge to Europe I am going over there and find out the truth.
I believe the Germans engineered these cars to go fast but the US government doesn't want us to know that. Can anyone say for sure what the mileage is for a 126 at say 130 mph on the autobahn?

OK let me have it.
Paul

PS all data came from gas tickets, speed and distance checked by gps and all calculations done on a spread sheet. So don't say I can't calculate, I didn't have to.

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84 500 SEL (307,xxx miles)

I would say low teens, as long as the pace was a steady 130mph.

FWIW - I averaged ~17.5 MPG in my E55 over 400 miles, with an average speed of 125mph.

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-Justin

91 560 SEC AMG - other dogs dd
01 Honda S2000 - dogs dd
07 MB ML320 CDI - dd
16 Lexus IS250 - wifes dd

it's automatic.

Who generally stated it?

You can't use a fixed number for all vehicles. It is a certainty that it takes more horsepower to push the vehicle through the air at a faster speed. This will consume more fuel. The only question is how much more fuel is required over the amount that is necessary to simply run the engine at that speed.

With a very economical engine that uses less fuel to run without a wind load, the wind affects the fuel economy in a significant way. My SD will lose about 10% on fuel economy if I change the speed from 62 to 75. However, the E38 doesn't seem to care if I keep the speed at 65 or run it at 80. I certainly know that it takes more fuel at 80 but the percentage is much smaller than the SD and the errors of fueling can easily lead me to a conclusion that it gets identical fuel economy at both speeds.

Fundamentally, I know that this is false and it consumes more fuel at 80 mph than it does at 65 mph, however it might be as small as 5% and I would definitely need to run at least 5 tanks consecutively to accurately measure 5%.

Otherwise, I might make a factually incorrect conclusion that it makes no difference in fuel economy if a large V8 sedan is run at 65mph or 80 mph.

i took my 96 bmw 740il out to california about 4 yrs ago.ran a consistant 23-24 all the way outto san berndino and then and back to lead colorado.i filled there and got on the interstate back thru nebraska and doing 85 that tank returned almost 29 mpg.don't know if it was just coming down from the elevation or what.next tank was also above the rest at around 26.after that it settled back into it's 23-24 range.one thing i liked about that car is the vacuum gauge.doing 60 or so gauge showed in the 18 range.doing 70 it showed in the 23-25 range or more vacuum.i would agree these german cars were made to open up and run.

The US Govt. publishes a book called the Transportation Energy Data Book (Oak Ridge National Lab) which has this data in it. Perhaps the table is helpful, but as Brian says, it is highly vehicle dependent.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

It cannot run more vacuum at higher engine speed. This would imply reduced airflow and it's impossible to run reduced airflow and get more horsepower.

Either the data is flawed or there was a gear change between speeds or the load on the vehicle was different between the two data points.

Well the 5% came from searches on the internet. Most sites say that 55 is the best speed for MPG. Some sites have graphs that show the MPG dropping for higher speeds. Like everything you read on the internet there will be 100 or more difference answers. But the fact is, my 500 will only get 20 mpg or more if averaging 70 plus. This is a proven fact and I have the spread sheets to prove it.
I think that the CD can be altered by the designers to tune the car for higher speeds. I know the engines can be designed for a specific HP and torque at a specific rpm range. So I don't think there is an answer to my question using readily available information. I am going to try some coast down hp test and see what happens. Maybe that will tell a story.

Paul

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84 500 SEL (307,xxx miles)

Like Brian said, no one number is correct for all cars. Mercedes spent a fair amount on wind tunnel research, so the drag coefficient is lower than on other cars, i.e. they are more efficient than some other cars.

However, the HP required to push anything thru the air is a geometric relationship---doubling the speed requires FOUR times the HP to overcome aerodynamic drag, tripling the speed increases the HP requirement by NINE times.
Also remember that aerodynamic HP is but a part of the complete picture of what it takes to move a vehicle on the road. Tire resistance, mechanical drag, friction in the drive train are some of the other factors.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

CD can be optimized to the degree chosen by the designer. A vehicle with a low CD will burn less fuel at 65 mph when compared with a vehicle that has a high CD. The vehicle with the low CD will burn more fuel at 80 mph than it will at 65 mph. The same will occur with the vehicle with the high CD.

However, the vehicle with the low CD will have a lower percentage increase in fuel consumption as the speed is increased as compared to the vehicle with the high CD.

As an example, if you run a Prius at 65 and again at 80, the fuel consumption might increase by 7%. However, if you run a W123 gasser on the same tests, the fuel consumption might increase by 12%.


Engines can be tuned a bit depending on camshaft and ignition timing so that their efficiency is higher at a given speed and load. It might be possible for a V8 to be more fuel efficient with a specific load at 2500 rpm as compared to 2000 rpm. Whether this increase in efficiency can offset the additional horsepower required to push the vehicle through the air is the real question. I would imagine that it's possible to optimize the engine performance to offset the additional horsepower necessary at a higher speed increase in a limited fashion. However, you will absolutely not get a vehicle to achieve the same fuel economy at 90 mph as it does at 60 mph. The required horsepower increase (3.375X) is simply insurmountable.

4X the force.

8X the horsepower.

Horsepower is a function of speed^3.

Yeah, I love those government studies. When they actually test ALL models of ALL cars and publish the results, it might be usefull. So many variables. Only constants are the laws of physics.
Torque curve of a particular engine with a particular exhaust system is the first place to look. In my experience, most engines are most efficient near the peak of the torque curve. But Gas engines cvan be pretty good on HP vs. fuel consumption at WOT. Lot of the cars in the college mileage competitions will run wide open then shut off the engine and coast. Pumping losses of course go way down at WOT. Turbo motors are of course a different animal.
Now throw in CD, frontal area (nobody mentions that), friction losses in the engine and driveline. And for real fun, throw in the slippage between the tires and the road, especially at high speeds. Then add in the type of road surface; conncrete or blacktop. Blacktop has more slippage- part of what you are driving on is oil (asphalt is part of the remainder after you crack oil).
BC made some important points about the percentages each factor plays in the equation. Really you would want to do a spreadsheet with all the different factors involved and then chart it for vechile speed. Then to have some fun, move around the rear end gearing and you can tune somewhat for what speed you normally drive.
I had a Datsun 260Z back when dinosaurs roamed the earth (so says my kid). Thing way petty low CD and had small frontal area. Straight six, normally aspirated with four speed manual and and about 3.56 gearing. Would barely crack 20 mpg at 55 mph if I was lucky. At 85 mph, it could do 26 mpg. How? My guess is the pumping losses went down with the throttle opened wider and perhaps the airflow through the whole thing just hit a sweet spot. Drag through the air still went way up but how big of a percentage of the first number was it vs the second number? Notice in the chart shown above, cars generally get WORSE mpg at speeds below 50/55. That optimum point just moves around for different cars. Mercedes lists fuel economy for different cars in some of their data books. My '84 190D is expected to get 39.4 pmg at 90 kph but on 34 mpg at 120 kph. Don't know what it gets at top speed, but with only 72 hp, it won't crack 100 mph anyway.
I'd love to see the whole map of fuel consumption vs engine speed vs throttle position for each of my engines. What year was the SEL? I'll see if it is listd in my data book and see what MB says it will do. For what market (US, Japan, UK, DE) was it built?

I've had a lot of cars and trucks and never NEVER had one that got better fuel economy going faster.

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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.

You never got better fuel economy at 25 mph than at 3 mph?

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

The car is a 1984 with a 85 engine from a car that came out of Mexico (I was told). I know it is a low compression engine that runs best with regular gas. No mixture control. So there are probably no charts for this combination.
I have read all this about doubling speed requires 8 X HP etc but no one has told me why my 84 500SEL will get better fuel mileage at 75 then it will at 55. Impossible is not an answer, because it is true. Just think, If I could figure out why this is true, I may be able to make it even better.

Paul

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84 500 SEL (307,xxx miles)

Impossible is the only correct answer. Sorry. You cannot change the laws of physics. I don't know if it was faulty recording, or a faulty measuring device but getting better fuel economy at 75 than 55 is impossible unless perhaps you had it locked into second gear at 55.

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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.