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  #1  
Old 09-22-2013, 06:32 PM
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2.47 rearend in place of 3.07s 123 300D

Has anyone gained fuel mileage from this swap?
I hàd a 83 300 D I just rebuilt trans. I do have a 3.87 first gear ratio now .not the old 3.68 so I have better take off now I can fell it. Was thinking of the 2.47 swap. It should take off better with my 91 560 gear set. I get 528 miles per tank w no ac on. W ac between 428 460 per tank
What u swappers think? Thanks

Getting 21-24 mpg on avg ac on. All city.never tried hi way as of yet btw

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  #2  
Old 09-22-2013, 07:28 PM
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I'm using the 2.47 rear end in my 617 powered R107 and am quite happy with the setup. A W123 300D is a little less than#200 heavier so you'd be getting a little less performance and a little less mpg. I'd assume that the coefficient of drag would be a bit higher as well. I get 27-28 mpg all around and up to 34 on the open road, so you'd be able to expect a bit less, all things being equal.

If you're looking for quick off the line performance you'll be disappointed, but if you're looking for quiet cruising and better engine longevity you'll probably be happy.
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  #3  
Old 09-22-2013, 08:52 PM
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I take it Baton Rouge is pretty flat -

gearing & city hills

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87 300D
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  #4  
Old 09-22-2013, 08:53 PM
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The 247 should be fine with an automatic. I agree with mach 4 around 30 highway and 22 in town I would estimate.
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  #5  
Old 09-22-2013, 09:22 PM
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I swapped my 560SEC's 2.47 for a 3.07 about 7 years ago. I didn't see a significant drop in mileage from doing that. The car gets 12-14 MPG in mixed driving -- about exactly the same as my stock 1990 560SEL.
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  #6  
Old 09-22-2013, 10:59 PM
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Quote:
Originally Posted by gerryvz View Post
I swapped my 560SEC's 2.47 for a 3.07 about 7 years ago. I didn't see a significant drop in mileage from doing that. The car gets 12-14 MPG in mixed driving -- about exactly the same as my stock 1990 560SEL.
Why did you do that?
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #7  
Old 09-22-2013, 11:01 PM
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lower gearing = speedier car. And about 0.75 second less than the 2.47 in the quarter-mile.

At 80 MPH, the car is turning about 800 RPM higher with the 3.06.
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  #8  
Old 09-23-2013, 01:52 AM
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You people are insane. 2.88 is *just* right.
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  #9  
Old 09-23-2013, 08:30 AM
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Quote:
Originally Posted by gerryvz View Post
lower gearing = speedier car. And about 0.75 second less than the 2.47 in the quarter-mile.

At 80 MPH, the car is turning about 800 RPM higher with the 3.06.
Well, you gained acceleration by gearing it so the engine has more leverage against the weight of the car. this causes the engine to turn more rpm on the highway or at any given speed, so unless you are driving a lot slower this will cause it to consume more air fuel mixture and unless the laws of physics have changed, will use more fuel.

If you changed the speedo along with the rear end you should be able to verify this by checking the mileage. the only accurate way is to:
1. check odometer against the highway mile markers so you can make corrections in your calculation.
2. Fill tank.
3. Take a nice long drive.
4. fill the tank again.
5. Divide miles by gallons used and you will have your correct fuel mileage.
6. Repeat three times and take the average.
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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.
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  #10  
Old 09-23-2013, 06:28 PM
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A minor change in axle ratio won't cause a major change in fuel economy. I think Gerry has already done the MPG calculations over time and found exactly that. My estimate would be ~10% difference, maybe.

FWIW, 2.88 vs 3.06 is splitting hairs... like, 1-2 tenths of a second 0-60 (or quarter mile) and likely under 1mpg difference in fuel economy.

The lower gearing makes it snappier around town, which is nice if the car is primarily city-driven.

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  #11  
Old 09-23-2013, 06:38 PM
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Quote:
Originally Posted by t walgamuth View Post
Well, you gained acceleration by gearing it so the engine has more leverage against the weight of the car. this causes the engine to turn more rpm on the highway or at any given speed, so unless you are driving a lot slower this will cause it to consume more air fuel mixture and unless the laws of physics have changed, will use more fuel.
Yes, but I assume that since Gerry's car got such awful mileage in he first place, he would not notice a big difference after a diff swap.
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  #12  
Old 09-23-2013, 06:45 PM
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Over the past eight years, and many dozens of tanks of petrol (not just three) I found ~1 MPG less overall with the diff swap, generally less. Again I had a 560SL and have a 560SEL (both with stock 2.47 rear ends) to compare with. The stock SEL (86K miles on the odo) gets about 1 MPG LESS than the modified SEC, and the SL (50K miles on the odo) got about 2-3 MPG BETTER than the SEC because of the size/weight of the car.

The speedometer difference due to the revised final-drive ratio was corrected with the aid of a "yellow box" converter, which is about the size of a cigarette pack and tucked in behind the speedometer.

The car still gets 12-14 MPG overall in mixed driving. That is still well within the original MBUSA specification for the car. Last Wednesday I drove it from Houston to Austin, TX and back, and it got 15.3 MPG at an overall speed of about 70-75 MPH.

I think the car takes more of a hit from the A/C running 9 months out of the year here in Houston, than it does from the revised rear-end ratio.

Hope this helps. Sorry that it's a reverse swap to what was originally mentioned. Note to the poster above ... the US-spec 500E used a 2.82 ratio, which is durned close to your ideal.

Cheers,
Gerry
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  #13  
Old 09-23-2013, 06:47 PM
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Quote:
Originally Posted by Jooseppi Luna View Post
since Gerry's car got such awful mileage in he first place
From the perspective of a diesel owner, this may well be correct.

However, my 450SEL 6.9 generally got 11-13 MPG, and my 300SEL 6.3 routinely got 8-12 MPG, depending on how many unsuspecting Porsche and Corvette drivers got smoked during that tankful. This was using 100 octane low-lead aviation gas, of course.
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  #14  
Old 09-23-2013, 07:37 PM
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What Gerry gives up in highway mpg might be offset by less throttle to accelerate the SEC at an acceptable rate. The OP will see an increase in highway mpg but maybe not in city or mixed driving if the pedal has to be floored to get across an intersection before the light cycles. If the car sees mostly city driving as the OP mentions, keep the 3.07 and accelerate less briskly (no one's asking, Gerry). Run a few tankfuls with 205/80-15s in the rear to simulate a 2.72 diff, if my my math is correct, vs 195/70-14s and a 3.07. Then decide if it's worth a diff swap.

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  #15  
Old 09-23-2013, 07:49 PM
t walgamuth's Avatar
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Quote:
Originally Posted by gerryvz View Post
Over the past eight years, and many dozens of tanks of petrol (not just three) I found ~1 MPG less overall with the diff swap, generally less. Again I had a 560SL and have a 560SEL (both with stock 2.47 rear ends) to compare with. The stock SEL (86K miles on the odo) gets about 1 MPG LESS than the modified SEC, and the SL (50K miles on the odo) got about 2-3 MPG BETTER than the SEC because of the size/weight of the car.

The speedometer difference due to the revised final-drive ratio was corrected with the aid of a "yellow box" converter, which is about the size of a cigarette pack and tucked in behind the speedometer.

The car still gets 12-14 MPG overall in mixed driving. That is still well within the original MBUSA specification for the car. Last Wednesday I drove it from Houston to Austin, TX and back, and it got 15.3 MPG at an overall speed of about 70-75 MPH.

I think the car takes more of a hit from the A/C running 9 months out of the year here in Houston, than it does from the revised rear-end ratio.

Hope this helps. Sorry that it's a reverse swap to what was originally mentioned. Note to the poster above ... the US-spec 500E used a 2.82 ratio, which is durned close to your ideal.

Cheers,
Gerry
Have you checked the odo before and after against the mile markers?

I got high teens highway with my 84 500 SEC with a 247 and running RUG. Best just under 20 mpg running about 65.

When you are getting 12 mpg a couple mpg translates to 20
% more fuel consumption. Taken annually it is significant. If you have a car getting 40 mpg to use 20% more fuel you'd have to get 32 mpg. One or two mpg on a very thirsty vehicle is a lot.

I don't wanna make more of it than what it is but the op is asking advice and it is a good thing if he gets accurate commentary.

Also I have swapped a bunch of diffs for mileage purposes and I have found that a 10% change in gear ratio on a diesel will net nearly a 10% increase in economy. With a gasser the results are not nearly so clear.

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..I also have a 427 Cobra replica with an aluminum chassis.
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