Yea I remember seeing that equation before as a check for people on the DSM forums who pull dyno runs out of their ace.

Here is a question that has been bothering me somewhat lately, say you could lower a car's aerodynamic drag just a few tenths. Then you need a certain amount less power while driving down the road at that same speed, however the motor is still spinning at the same speed. Would the fuel mileage really show any difference or would there just be more vacant power? I suppose in our MB instance it would be the difference between the turbo just spooled and boosting a couple psi, so the lack of need for this fuel would make a difference, right?

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

The primary requirement for horsepower at highway speeds is wind resistance. Most of the horsepower is required to overcome the friction of the wind over the body. A fraction of the horsepower is required to run the engine at the desired RPM, independent of the wind. With a gasoline engine, being throttled, it is inefficient except at maximum power. At 55 mph, it puts out about 15 hp on level ground and it has the capability (with the latest rice burners) to put out nearly 200 hp. So, in this instance, if you lowered the wind resistance slightly, you would get a very slight benefit.

However, in a diesel, there is no throttling and there are no pumping losses, so the engine can run at 3000 rpm on very little fuel. Most of the fuel consumed at 3000 rpm is used to overcome the wind resistance. If you were able to reduce the wind resistance, you would get a noticeable increase in fuel economy. This can easily be seen when comparing the fuel economy of the 603 engine. In the 126 body, the fuel economy is roughly 25 mpg. The same engine in the smaller 124 body gets over 32 mpg, as evidenced by the discussion from those who possess these vehicles. The only difference is the reduced friction of the wind over the body.


Brian Carlton
1984 300SD

Brian, your full of interesting information but I suppose it wouldn't be appreciated for me to hijack this thread so I emailed you if you don't mind.

Tscott, the kind of overdrive you were actually wondering about was the 'one-way' clutch kind, or the extra gear kind? Or both?

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

You can quickly check this buy falling in behind a semi. You usually have to lift on the pedal to maintain the same speed.

Boy, that sure came out of the attic, LOLOL.

If you have the nerve to get really close to a semi, like within six feet, he will tow you along with your foot off the throttle completely. I do not condone this behavior, however.

Yeah, read the entire post after following a link in another post to it.

Not that close, but noticed on a recent trip with a 240D (now in my daughters hands) just how much help a semi can give.

I would not get that close either, without permission from the driver. Sometimes he will agree. Most times he will not even respond to you

I try and stay far back enough that they can get a glimpse of me in a mirror. This is still close enough to make a difference in where the accelerator pedal is. Sometimes, drivers do not like a vehicle in their mirror and if they change speed, etc. then I will assume this to be the case and not follow.

Speaking about rolling resistance, I don't remember any car I've owned that the brake disc pads didn't touch the rotors to some small degree, without breaking of course, and I've wondered there must be some loss in fuel economy as a result!

My other car began loosing 3 or 4 mpg when I let a bad front wheel bearing get worse, could barely turn the wheel while jacked up! After it was fixed, my fuel economy returned to normal!

After a 1000+miles of vacation travel last week in the 300D, much of it spent at 80-85mph I was thinking the same thing. I don't want to drop the rpms down below 3000, but 3300-3400 would be nice. Maybe a diff from an 85 and some slightly taller tires would be a better option? RT

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When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K

The '85 diff will reduce rpm's by 6.5%. If you run 215-70-14's you can reduce rpm's by another 3%. I've run these exact tires on the 123 without any issues. So, you could get a total of 10% from this combination. That would certainly help the situation, without losing too much horsepower.

Remember, however, that each time you go to taller gearing, whether with tires, or with gears, the exhaust temperatures will go up accordingly. You need more torque from the engine to maintain the same speed because rpm's have been reduced and horsepower is constant.

An overdrive gear would not be desirable unless you were running the vehicle on totally flat terrain.