Rear end swap

[danonly55]

I am trying to get more top speed with less rpm in my 1980 300sd, I was thinking of maybe a 450sel same era rear end swap. Will this do what I need or what do I need to do ?
Thanks , Daniel

[Grzpdlr]

Originally Posted by danonly55

I am trying to get more top speed with less rpm in my 1980 300sd, I was thinking of maybe a 450sel same era rear end swap. Will this do what I need or what do I need to do ?
Thanks , Daniel

One year only, 1985 MB 2.88 diff, rare but available

[Mike D]

Do a "search" on the board for information.

M110 Engine Identification in my '74 w114 Sedan

Post #10

As GRZPDLR notes, the 2,88 will be the "lowest" ratio which will swap into your existing differential housing (185 MM). You already should have the 3,07 so I doubt you would see much improvement.

Numerically a 5% improvement but some of that will be lost in a lower efficiency at lesser rpms. Kind of a trade-off and it all depends on percentage of highway speed vs. in town use.

[fonzi]

If this is all about the ratio, then figure out what ratio you currently have first. It should be marked on the Diff. It’s an old car. It should have the originally optioned ratio, but you never know what it has now until you check.

[Usaguy]

Originally Posted by danonly55

I am trying to get more top speed with less rpm in my 1980 300sd, I was thinking of maybe a 450sel same era rear end swap. Will this do what I need or what do I need to do ?
Thanks , Daniel

Doubt you will get a higher top speed.

450SEL had a 3.07 or 3.06 axle except for the 1980 model year.

Also I would trust mb engineers who designed this car and leave the axle stock. They knew that a taller axle would get them better fuel economy and less noise and yet they decided to stick with a 3.07 ratio

[Grzpdlr]

Originally Posted by christuna

Doubt you will get a higher top speed.

450SEL had a 3.07 or 3.06 axle except for the 1980 model year.

Also I would trust mb engineers who designed this car and leave the axle stock. They knew that a taller axle would get them better fuel economy and less noise and yet they decided to stick with a 3.07 ratio

The problem with this logic is that, at that time, the speed limit was 55mph, they had no idea we would be cruising at 75mph

[Mike D]

Only in the U.S. was the speed limit 55 MPH.

[Usaguy]

Originally Posted by Grzpdlr

The problem with this logic is that, at that time, the speed limit was 55mph, they had no idea we would be cruising at 75mph

No

Originally Posted by Mike D

Only in the U.S. was the speed limit 55 MPH.

Exactly.

In Germany where there were/are no speed limits in many places Mercedes used the same axle ratios as in the US

[Benz Dr.]

Originally Posted by Mike D

Do a "search" on the board for information.

M110 Engine Identification in my '74 w114 Sedan

Post #10

As GRZPDLR notes, the 2,88 will be the "lowest" ratio which will swap into your existing differential housing (185 MM). You already should have the 3,07 so I doubt you would see much improvement.

Numerically a 5% improvement but some of that will be lost in a lower efficiency at lesser rpms. Kind of a trade-off and it all depends on percentage of highway speed vs. in town use.

I've always thought that small numbers meant high gearing and big numbers meant low gearing. A 2.88 is a much higher gear ratio than a low ratio 4.08

[Mike D]

Yeah well, that's that fuzzy differential logic. A rough way to look at it is:

A "higher" differential ratio means the pinion gear needs to revolve more to turn the ring gear once. It takes 4,8 revolutions of the pinion to turn the ring gear once. This results in higher RPM's of the engine but lower revolutions of the ring gear/axles.

Kind of like how when you are in fourth gear in a manual transmission. The input shaft is turning at a 1:1 ratio with the output shaft resulting in lowered RPM's but in first gear the ratio is more like 3,75 to 1.

[fonzi]

In a 2.88, the drive shaft rotates 2.88 times for every one revolution of the axle or wheel.

If you are only looking to change from a 3.07 to a 2.88, why don’t you just put taller tires on? Get a tire that’s 6.6% taller than stock, and you effectively are doing the same.

205/70/14 —> 225/75/14 or 225/65/16 and you are getting 7-8+% more per drive shaft rotation.

And it’s a lot easier than swapping the diff.


Sent from my iPhone using Tapatalk

[t walgamuth]

Originally Posted by Benz Dr.

I've always thought that small numbers meant high gearing and big numbers meant low gearing. A 2.88 is a much higher gear ratio than a low ratio 4.08

agreed.

[Tomguy]

Originally Posted by Benz Dr.

I've always thought that small numbers meant high gearing and big numbers meant low gearing. A 2.88 is a much higher gear ratio than a low ratio 4.08

This is how most enthusiasts look at it.

For example, a "Low" first gear provides a lot of grunt but tops out at maybe 3, 5MPH in a big rig.

A high gear ratio is numerically low, but it allows for a higher top speed. I think this is where the high versus low comes from (your wheel speed is high or low).

Think about 4x4s with "4 LOW" where your gearing is maybe 4:1, versus 1:1 of "4 HIGH"

Anyway, I would suspect a 300SD is geared the way it is for a few reasons, one of which is potentially aero drag and horsepower. Would a taller gear allow you to go faster, or would the drag of the brick, combined with its lower horsepower, result in insignificant changes in top speed with a stiff penalty to acceleration and on-ramp performance?

[t walgamuth]

Back when I had my 240D with 300D na euro motor with the 307 diff and five speed the car would run 100 mph in either fourth or fifth.

(for what it is worth).

Where the taller gear helps is mpg which that car would do 40 mpg if driven 60 to 65 mph.

[rjz5400]

i have a 2.88 diff for sale in the parts section if you decide to go that route.