The 2.88 rear end is now installed in the wagon (replacing the 3.46's). Overall not a frustrating project, but it sure gobbled up the hours. But then I cleaned all the parts inside and out before putting it back together.

Only drove it a couple of miles to check for leaks and proper operation...but I have to say it has made a very positive change in how it drives. The shifts are smoother, the engine not racing at highway speeds.

I suspect mileage will go up a fair bit as well.

Glad I went through it

Don

__________________
1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

You mean it is not a slower turtle than it was to begin with?

I will be very curious to see your fuel mileage results. I will be very surprised if it makes a significant change.

The most significant factor in the fuel mileage equation is weight. The only way changing gear ratio will have any significant fuel mileage difference is if you move the RPM at which you frequently drive closer to the torque peak and you cruise alot. Even then it will be close to immeasurable.

If you drive mostly stoplight to stoplight, I would expect your fuel consumption to increase.

Good luck,

Took it on a longer drive yesterday. It is sooooooooo nice to drive in comparision to the old gears. I live just outside of Yosemite and all the roads are highways. 1st stop light is about 12 miles and the one that follows that another 20. The speed limits on all the major road range from 55 to 70, with minor roads at 40 and 45

To be silly...If I had my gearing set so that at redline I had a top spped of 10 MPH in case 1 and top speed of 100 MPH at redline in case 2, logic says case 2 will get better miles per gallon, agreed same gallons per hour.

In my real world case, I have about a 20% reduction in RPM and therefore I am "less into the pedal".

Not saying I am am expert...just discussing logic...

Would it be fair to say that your feeling is that had I driven my wagon before the work at 40MPH, the mileage would be the same as 50 or 50 as compared to 60 and so on? (trying to learn not being a wise guy).

Don

__________________
1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it

I'm not trying to debate or argue here.

If increasing fuel mileage was as simple as using higher gearing, every car in the world would be using 1:1 gearing or something. Although the engine doesn't turn as many RPM, it must deal with the load. That means that you will be pressing harder on the throttle, thus injecting more fuel in the cylinders to accelerate. You will also have to press harder on the throttle to maintain cruise speed.

It's not entirely about the number of times the cylinders fire in a mile, but how much fuel flows through the injectors during that mile. When going to a higher gear in most cases you will have more load, thus require more throttle application.

The most efficient RPM as far as fuel usage vs. torque developed is at the torque peak. I don't have a clue at what RPM that falls on your engine. If you can move your cruise speed close to the torque peak RPM, you THEORETICALLY have the ideal gear ratio. There are limits to this, however. If you have some super high speed engine with a torque peak at 5,000 RPM, your not going to get very good fuel mileage by gearing such that the engine screams along at 5,000 RPM to cruise at 50MPH.

Now. In some cases having that ultra high gear will indeed provide less fuel consumption even though you add load, you also decrease number of firings, but this will ONLY occur at steady, level ground cruise situations. If most of your driving is cruising on level roads, then it may very well offer a fuel savings. If your driving is stoplight to stoplight, it will take more throttle application to accelerate the car, thus use more fuel.

All this is theory and we all know that theory does not always work out in practice. There are ZILLIONS of variables in fuel mileage, not the least important of which is driving style. If you were to drive lots of stoplight to stoplight, but your driving style inolves VERY light throttle application and not being in a hurry, then you might even find an increase under those conditions.

Good luck,

Fuel = horsepower
Regardless of the gearing it takes so much fuel to develop the horsepower to move the car. You can spray it in in one large shot per firing or several small shots over several firings. The best way to get better mileage is to reduce the horsepower requirement (weight, drag, ect) or raise the efficiency of the engine.

__________________
1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C

Ok men, real life situation.

I have been reading posts on the rear end swap and the theoretical arguments on mileage and I can't entirely disagree with the points that have been made. However, the little lady's 300SD is a screaming banshee at 75mph. Our driving is on relatively flat roads, at 65mph, with maybe one or two stop signs every 30-40 miles, so the swap makes sense from a comfort and driveability standpoint, if what I have read in previous threads is somewhat accurate.

But I was curious about the mileage and performed a (limited) test. I had occasion last week to make the same 250 mile trip on two different days. I drove the Buick.

On the first trip I drove, as is customary, in overdrive, with tach at 1,550 (63mph) and mileage was 28.6.
On the second trip I left it in 3rd gear for the duration, with the tach showing 2,300+ at 63mph. Mileage was 20.2.

So - according to what I have read, changing from the 3.07 that is currently in the 300SD, to the 2.47 that is currently waiting to be installed, should drop the rpms by about 800, which is roughly the difference between 3rd and 4th in the Buick.

I can see no reason why I won't see a similar increase in the mpg in the SD, which should push it well over the 30mpg mark.

With 1/2 an ounce of luck I will swap the rears this weekend and will then track the mpg and post results.

Jim

I'm with Larry on this one, I don't think there will be a significant mileage change. The car still has the same amount of drag drving down the road at highway speeds and that's what's going to be eating up fuel once you have it moving. I'm sure it is much more pleasant to drive now that the engine isn't screaming.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k

Now this is an interesting thread. I agree with most statements made above however I am aware of one proven gear swapping scenario that does change fuel mileage significantly: Diesel pickups, specifically GM's. With the old 6.5TD engine you could expect 13-14mpg with 4.11 gears, 16-17mpg with 3.73's and 19-20mpg with 3.42's. (running empty) Owners regularly swap gears in search of better mpg's or better towing ability. The mileage gains/loss are documented and well known. Obviously the vehicle is much larger and heavier, between 5500lbs and 7500lbs empty for most. The engine is much larger too. Since it is known that 617 engines in MB's don't ever seem to deviate much from 25-26mpg, regardless of gearing, I wonder why there is a difference in the trucks? Sure its a strange comparision, but you gotta wonder why. RT

__________________
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

If you change the range of engine RPM to complement the efficency of the engine you will get better mileage. If you lower the drag you get better mileage. Gas engines are different than diesel because they have a narrower torque range.

__________________
1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C

Before my rear end change I got 26 mpg every time, validated with GPS. After the change, my mileage went to 28 mpg average with 29 and 30.6 being highs. I even got 25 on a 1300 mile ski trip in February where I cruised between 90 and 100 for a good while. I generally average 80mph on the road and my miles are 40% highway, 60% in town. None of these numbers were with A/C. By the way, I could never stand to drive that fast with the 3.07.

The other monetary consideration is that a rear end change will make your life less stressful and that saves medical expenses later on. All of this assumes that you don't hurt yourself during the swap...

I have read an engineering book but can't find it where it said that an engine's most efficient speed is at it's torque peak. 2400 rpm is our engines' torque peak, not 3500 like the MB marketing department wanted. If MB had geared these things with 3.46's they would have had a hard time selling 380 SE's and SEL's. All of this is only my opinion.

Jim
1982 300SD
Getting an R12 restoration like God intended...

RT, I have heard the same regarding the GM's.

It seems to me, and I may be upside down on all of this, that as long as you have adequate hp/torque, the increase in the fuel consumption is marginal as load is increased, within reason, at a given rpm; most fuel consumption appears to be dictated by rpm.

Example: I have an 80hp farm tractor that burns 2.2 gph at 1500 rpm, or 3 gph at 2300 rpm under light load. However, it only burns 3.2 gph under heavy load at the rated (PTO speed) 2300 rpm.

This tells me that it takes a given amount of fuel mostly to maintain the rpm. Translate that to the auto; as long as the 617 has the balls to push the car 70mph in the mid 2000's without working too hard (which appears to be the case), it should burn considerably less fuel than it would turning in the high 3's.

Jim

dieselgeek,
What ratio did you swap to? I have a 2.88 to replace my 3.07 in the '84 300D. Not expecting a mpg increase just a little less noise on the highway. I agree that the gearing should put the engine at its torque peak at the desired cruising speed. Lets not forget the speed limit was 55 when these things were sold. My 300D seems happiest at 60-65mph. Sure it will run all day at 80mph, but seems to feel best right at 60-65. This is about 2900rpm, much closer to the torque peak, which would seem to make sense. A lockup torque converter or a 5speed would be a really nice thing to have behind a 617. RT

__________________
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 SD, which has the same 3.07 ratio, with larger tires (215/70/14) is happiest right at 68 mph. This matches precisely to your 300D at 65 mph, RT.

My thought is that these vehicles feel right based upon rpm. If they could turn 2800 rpm at 80 mph, then they would be perfectly happy with that.

This is why the conversion to the 2.47 is so attractive for folks who routinely cruise at 75-80.

i agree that the mileage for a 617 with a 2.47 should be over 30 if driven at 75 mph. in my 240 with 300 non turbo and 3.08 gears i get 30 consistantly at 75 mph. the engine pulls it easily and it is very quiet. with the 3.08 taking off is noticably less speedy though.

__________________
[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 used a 2.47 from an '84 380SE...2900rpm at 80mph. Itsa quaalude on wheels!

Jim