This may have already been discussed, but I am curious what you guys think the best highway speed is for gas mileage. My guess is that it would be around the speed when your car shifts into the highest gear. Do RPM's vary directly with fuel consumption? i.e. If running 2k rpm uses 2 gal. an hour, would running 4k rpm use 4 gal. an hour?

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'98 C280

Birmingham, AL

I believe this has been discussed earlier but as I understand it the optimal MPG's is achieved where the engine is working just above the maximum torque in the highest gear which I guess is 2400RPM's for most of the MB cars. There are a lot of other factors that affect mileage the biggest other than the engine itself is the wind resistance which increases at the sqaure of the speed and not linear so you have a 4 times the wind resistance at 60mph than you do at 30mph. The diesels aren't designed to run an extremely high speeds either. The fuel's combustion rate is slower than gasoline and therefore it's efficiency diminishes as the speed increases. You can see the decrease in performance when looking at the horsepower and torque charts.

Of course the best way is to fill the car to the neck and drive it with the CC on a road trip and see what it gets for each leg at different speeds.

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

You wouldn't last a day in New Jersey. You would be run off the road within minutes When the sign on the highway says 55 you should not be doing below 75.

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Audi TT

if i drive 71 or 72 somewhere in there at about 3K rpms, I get 250 / half of a tank. pretty good, and I haven't found a better speed yet. Although i imagine a little slower might make it better.

When I first got my Benz I started experimenting imediately with different speeds to see where it's sweet spot was for mileage. My worst mileage was at 55 and it's best was between 70 and 75. As far as Stay alive, drive 55 . I want the best fuel economy with today's high fuel prices.

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1983 300-D turbo
1985 300-D turbo
1959 Harley Panhead chopper
1929 Ford coupe restored
I hang out with Boneheaddoctor at Schuman Automotive OBK#5
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The best highway speed for gas mileage is a balance between engine efficiency and wind efficiency.

The best speed for wind efficiency is zero. If the vehicle is not moving, it's doing no work to move the air aside.

However, the engine is consuming fuel a zero speed. So, this is hardly the best speed for the highest efficiency.

Somewhere around 40 mph is a point where the wind resistance is fairly low and the engine is not doing too much work to move the air aside. So, in reality, it is consuming just slightly more fuel than it otherwise would at zero speed.

However, the rpm's of the engine are higher at 40 mph, so additional fuel is required to achieve the higher rpm's.

Above 40 mph, the engine consumes significantly more fuel for each 10 mph increase in speed, and, additionally, must overcome the higher friction of more rpm's.

It's possible to achieve the most efficient speed at 45 mph if the vehicle is geared tall enough and the rpm's are low enough. The additional work to push the air aside is offset by the lower engine speed.

Anything above 45 mph will result in lower fuel economy.

The correct answer is: "Zero" MPH.

Park it and take a bicycle.

I just drove a whole lot this past week (2k miles) in my W124 E320 and averaged 26 mph. I usually drive about 10 MPH over the speed limit on the interstate and 5 mph over on all other roads. Except school zones. I don't mess with school zones. Oh yeah, I also slow down near police stations.

Botnst

Bot, you forgot work zones. I think they are even more vigilant around those areas than they are around school zones, and they show NO mercy.

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