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Old 05-09-2003, 11:04 PM
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Gear Differential Question

I have done some research through over 50 pages of posts about changing gear differentials, but i still have a question.

Here is some background info:

I have a 77 Euro 350SLC with an automatic tranmission and a 3.46 differential.

The low end in the car is so powerful, i can burn rubber off my 8" rear tires. The acceleration is enough to pull my head back.
Not bad for a 20something year old car!
Unfortunately it takes 3000rpm to go 60mph, 4000rpm to go 80mph, 5000rpm to go 100mph, you get the idea.

I want to be able to get better gas mileage and to be able to go at high speeds without having to put such a high strain on the engine. I don't mind sacrificing the low end performance.

A person from a well known mercedes dismantler shop told me all 77 USA 450SL / SLCs came with a 3.07 differential as standard equipment. He said i may no see much of a difference installing a 3.07, and he offered me a 2.47 differential from newer 500 / 380 SLs

The question is: which differential should i install?
3.07? or 2.47?
Will i cause any damage in the long term by installing the 2.47?

Any help in this matter will be appreciated.
Thank you all for this great forum.

Lovingly rebuilt 77 350SLC
Northern NJ.
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Old 05-10-2003, 12:36 AM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,010
You can determine the new speed/rev relationship by multiplying revs by the ratio of the new to original axle ratio.

For example, if you are turning 4000 revs at 80 with the 3.46, the revs with the 3.07 will be:

4000 (3.07/3.46) = 3549,

about a ten percent reduction.

The SLC is a fairly heavy car, so with only a 3.5L engine it needs short gearing. U.S. spec 450s can pull a taller gearing because they have more torque.

If you were to make the change I would not go "taller" than the 3.07. (The 2.47 is much taller.)

The 3.07 will reduce acceleration, but it will probably be liveable. With the 2.47 you will notice a considerable loss of acceleration.

The other issue you want to investigate is the speedometer. Changing the axle ratio will upset speedometer calibration by the same ratio as stated above, so you need to understand what changes must be made to bring it to proper calibration with the different axle ratio.

The 3.07 gear will probably not provide more than 5 percent better fuel consumption, and wear is not that much of an issue. Higher revs means lower torque required to maintain speed, so the difference in engine longevity between two gear ratios ten percent apart is not significant.

The biggest reason to change gear ratio IMO is to reduce engine noise at freeway cruising speed, but with a V8 the difference will not be that much.

Bottom line, don't set your expectations too high for what you'll gain, and for what it will cost, it might not really be worth it.

One other observation. Assuming your top gear is 1:1 with a 3.46 ratio your tire revs per mile would have to be 867 revs per mile to be truning 1000 revs per 20 MPH. If I recall correctly your OEM tire size is 205/70R-14, which are about 813 revs per mile, This leads me to the conclusion that you have either a shorter (high numerical) axle than you think or your speedometer is reading close to ten percent low.

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Old 05-10-2003, 01:04 AM
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Originally posted by Duke2.6
...The biggest reason to change gear ratio IMO is to reduce engine noise at freeway cruising speed, but with a V8 the difference will not be that much.
Hi Duke
Thanks for all your help!
The high revving to attain high speeds is the main drawback of my benz.
I did some math and by installing the 2.47 differential, i will be able to do 60mph at around 2200rpm.

Both of the previous cars (Volvo, Mazda) i've owned, used to do 60mph at around the same rpm.
As i said before, i am willing to sacrifice some of the acceleration for fuel economy and longevity.

Could anyone with a late 70s 450SL / SLC tell me at what rpms they do 60mph (at the highest gear)?

Thanks again
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Old 05-12-2003, 05:36 PM
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Could anyone with a late 70s 450SL / SLC tell me at what rpms they do 60mph (at the highest gear)?

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