Mercedes Benz 240TD
http://mars.ark.com/~dcf/240td.html

Mercedes-Benz Heritage: 240TD
http://www.cwwcardesign.com/cars/240td.html

Thanks Marty.........I need to follow your lead.........

Can you install a higher rear gear in a 300SD to get better highway mileage?
At what point would it be counter productive?
Where can you source the gears?
Thanks,
Greg

I agree. My former diesel (a 95 E300D) experienced better fuel economy if I drove in the 60-65 range than any higher speeds. In fact, every diesel and non-diesel car I've owned -- from Ford Festivas (there was a great little fuel saver) to a Ferrari 348 (not so much) -- always gets better fuel economy if you stick to the speed limit though, like NHdoc says, its very difficult to do in any populated part of this country. If you can get away with driving slower though, its actually kinda relaxing, quieter, and you save money too.

I guess I'm starting to show my age with statements like that...

That is exactly what I'm telling you, you don't have to believe it if you don't want to.

Yep, thats what I have noticed as well. But I want to drive it like a MB on the highway, so I'm rarely under 80. Fuel ain't that expensive!

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Ok, its my turn to chime in, as you know I have had a 2:47 sitting around and finnally got her installed 2 weeks before Christmas. Thats a 2:47 swap with a 3:07 in my 84 300d. Took it to Key West from Long Island over the holidays and boy was that swap ever worth it.
The ride is great, with the drown of the engine spinning at 3450 at 75mph before the swap, is almost undertectibale after the swap. As best as I can figure by using the mile markers on the interstates and timming with my watch so I can figure my correct speed, I was turnning approx. 2950rpm @ 75mph. The odometer correction as best as I can get it is 2 tenths off per mile. I used those figures into computing mpg's for the trip.
Well the first leg from LI to North Carolina was great. Speeds 70mph were constant and I filled up in North Carolina after the low fuel light came on and took on about 16.3 gals, and that was sqeezing it in, drove 498miles=30.55mpg.
That was with a/c on the whole time because of the dampness from the rain.
The best mpg for the trip was from Florida City, The closest town before entering the Keys, round trip to Key West back to Florida City, cruising with sunroof open and and windows slightly down, keeping the rpm's around 2000-2500 not going over 60, mostly 50-55, driving around Key West stop and go for thee days and filling up back at the Hess station at the same pump and the car in the same position, 300 miles round trip 9.06gals=33.11.
As you can see I have some increase in mpg, and like Brian says about your speed, mpg falls because of higher wind drag.I can tell you my other tanks where not bad either but not over 30 again. Mostly around 28-29 at 75mph with a/c on and also girl friend in car also and all her extra crap in the trunk.
My worst was under 26mpg and It was the fuel quality, I mean I had to put more of my foot in it to get the same speed, you can definitly tell the differences in fuel quality right away and you know you got ripped off after 10 miles down the road and you can't do anything about it.
I also want to mention about the hot fuel issue, which I did get some down south, like I did out west, one fill up the fuel was 75 degs.You can tell easaliy as you are filling up as the nozzel is warm and the surrounding air is 60degs or so. This also affected my mileage I thought on that worst tank of fuel. And yes I did have a thermometer with me that I use for a/c monitoring.
So if you want to do a 2:47 conversion, I highly recommend it as it was so far the best upgrade I have done and the car behaves great around town. I will have the city mpg averages soon as I have to go thru a couple of tanks of fuel.
If anybody has any ideas on how save more fuel please share them on the forum, they will help every body that has a mb diesel.

Peter
1984 300d 280k

Actually, yes, because wind and rolling resistance increases proportionally to speed squared so the resistance at 70 MPH is 36% higher than at 60 MPH but the speed is only 17% higher, so you see you will actually consume fuel at a much greater rate at 70 than at 60 which will more than offset the time savings. Even if though an engine has an optimal efficiency speed (RPM) it would not overcome the additional energy needed to overcome the resistance at higher speeds to move the car.

MPG doesn't care how fast you go, it is simply a measure of how much fuel you consume to cover a specific distance; basically it is the energy derived from the fuel. You will always require more energy, and hence consume fuel at a faster rate than your speed increases as you speed up. If you had driven a car with an MPG gauge you can see it very clearly that you will always get better MPG at lower highway speeds than at higher ones, period. There is no "sweet spot" that magically will cause your car to get higher MPG at 70 or 75 MPH than at 60-65 - it is just more internet folklore from people who don't understand physics. The same people who believe that adding 2 ounces of acetone to a tank of gas will increase their MPG by 20%.

Flame on, crackpots.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

I think they were only available in Europe or Germany.

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'83 300D Turrrbo 295K miles

There are just so many variables in real world mileage. A classic example is me driving or my wife driving. I get up to speed and go on cruise control, I look waaaay down to road for slower cars or stop signs, I coast up to stops, I drive real smooth, I try to never hit the brakes, time red lights so I can just keep rolling. I get better mileage out of every car we've owned.

Now the red head get in, fires it up, races the engine a few times to get it "warmed up", slams it into gear, and floorboards it! She races up to red lights, afraid they will turn green before she gets to come to a sliding stop...which always bothers me, because I wonder if she has even seen the red light and btw, it bothers other drivers because they are afraid to move because they don't know if she plans on stopping either. On the highway, she'll go 50 or 60 miles before she remembers to turn on the cruise, which she sets for 80 mph! If a slower car is in her lane, she will run right up to it, slam on the brakes, then check her mirrors to see if she can pass, then she floorboards it to whip around and get back in her lane only to have to....you guessed it, slam on the brakes AGAIN! In town, she must race up to the next red light...she just loves hauling ass and sitting still...I don't get it. Anyway, her mileage ALWAYS just sucks and don't get me started on how to ruin brakes, just see the above.

However, in the real world, we both get better mileage if we just slow down a bit. As a side note, even with her more aggressive driving style, we always arrive within 3 or 4 minutes of each other...she always says "I win" and I reply "Someday you'll lose it all".

My point is this, these are big heavy cars, with the aerodynamics of a Sherman tank, hence fuel mileage that sucks, period. Want great mileage, then drive a different car. If you want better mileage out of your old MB, get smoother as you go slower!

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Mark
1983 300TD Wagon
Even a broken watch is right twice a day

Why does changing the gear ration in the differential affect the odometer mileage? I understand that using a larger tire could affect this, but how does the gear ratio change in the rear end affect it? Thanks.

Because the speedo and odo are based upon the speed the transmission output shaft (or driveshaft) is turning and it assumes the stock rear end is installed when it is calibrated. If you put a rear end with a lower ratio the speedo will read lower because the output shaft is turning slower to go the same speed at the wheels as it was with the higher ratio rear end. In this case, he'd read 80.5% of what he's actually doing because the new ratio is 80.5% of his old one and his engine RPM at the same speed should be 80.5% of what it was with the old rear end.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

What are you going to do about the whole speedometer/odometer issue? Is there a correct W123 unit for a 2.47, or have you found someone to re-calibrate it? I've thought about changing my gearing (probably after I eventually have to do my engine), but I'm not interested unless I can make the speedometer/odometer correct. Otherwise, I might just toss in a 2.88 and use the instrument from a '85.

Toby,

After owning an '83 300SD to 305K, my data was 30 mpg highway @ 65 mph. City varied on this car between 24 and 26 mpg.

The '99 E300TD is all over the place on mpg. It delivers a solid 32 mpg @ 65 mph. And between 22 and 28 City mpg. Combination mostly HWY & some CITY never yields greater than 30 mpg.

That SD was some car in many ways much more a car than the '99 E300TD.

So how do I tell what gear ratio my 85 has? and the 83? where would I get the 2:47 ratio? and although milage went up, what about power loss whan climbing hills.

Would I just have a shop change the gears?
this sounds like a good idea on my daily driver, as it is 35 miles each way....

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83 300SD Dark Silver Dark brown inside