The table above outlines the various W108 and W109 differential ratios.   For a number of models, there was a change part way through production.   Where possible, the chassis number break point has been identified.   This is not foolproof today, as the cars are now around 50 years old and may not have their original rear axle.    To make it easier to read, the table separates the standard version from ‘special versions’.   Cars with the optional 5 speed manual transmission, optional 15″ wheels or bound for the USA generally received shorter differential ratios.

The optional 5 speed was used to increase performance as well as a modest reduction in highway RPM.    That is why W108 and W109 differential ratios for 5 speed cars are shorter.   It is worth noting that the torque peak for most of the sixes, and even the 3.5 V8 is around 4,000RPM.  The ratios therefore align the characteristics of the engine with a reasonable cruising speed.   Even the long stroke M189 engine has a fairly high torque peak.

The USA ratios are interesting for the six cylinder cars.   I can only assume that since most domestic vehicles in the 60s were equipped with V8s, Mercedes-Benz North America felt it necessary to order the cars optimized for acceleration rather than cruising.   Certainly this is not appreciated by modern American owners, who find the setup of the cars does not really match their interstate cruising speeds.   Rear axles from wrecked 4.5 liter cars have been a popular swap.

Looking at this table, it seems clear that Mercedes-Benz was learning as they went.   The change to the M130 2.8 liter engine introduced a nice increase in torque from the old 2.5 litre unit.   It took them a  year or so to adjust the W108 and W109 differential ratios accordingly.   The same went for the M116 V8.   The 300SEL 3.5 was introduced about a year before the 280SE/L 3.5 with a 3.69 ratio.   This was amended down around the time of the introduction of the 280SE 3.5 which was always sold with a 3.46 ratio.    There were also changes for production reasons, such as the 6.3 and 4.5 models.

The other factor here, is that the W108 rear axle has been a popular upgrade for owners of the W111 models.   Those models to 1965 had a drum brake rear end.   The W108 provides disc brakes and more choices of ratios.   The W109 rear axle is different due to the air suspension.    While it is possible to change the whole axle over, some people have swapped the crown wheel and pinion.   Tall ratios are now quite expensive as supplies dry up.

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In compiling the table below, I have tried to research the various country specific variants.   It is possible I have missed a few.  Please indicate in the comments below if that is the case.

The SL and SLC of the year had the same mechanical specification (at least during the SLCs production run).   The only exception was the 450SLC 5.0 where there was no equivalent 450SL 5.0 until the facelift and the 500SL model.   

Many of the same aspects that were covered in the W126 table are applicable here.   As with the W126, the V8 models received the larger (1.3l) case.    Also, as with the W126 models, Mercedes-Benz went on a fuel efficiency drive during 1981 which is why the V8 cars got revised ratios.   The project was called the “Energy Program” and included other changes such as the revised Bore/Stroke of the 380 engine.

Early in the first generation production, the 450 models moved from a 3.07 ratio to 3.06.   This was achieved by changing from a 46:15 gear set to a 52:17 gear set.   This was more of a minor specification change than any real impact on the car.

From the long term ownership of a 450SLC, I think the chosen ratio works well for that car.   I’m not as sure of some of the other ratios.   My view is that some of the later ratios were too tall.   They probably worked a bit better for the home market on the Autobahn than in countries with lower speed limits.

The other factor to take into account is that this generation of Mercedes-Benz automatic transmission did not feature a lock-up torque converter.   This means there will be some level of slip and engine revs will be slightly higher than the theoretical number.

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There are generally two ways of accounting for the differential ratio in the speedometer.   American cars have an easily changeable speedometer gear in the output of the transmission.   This allows them to have the same speedometer regardless of the differential ratio.   This approach simplifies things for their high production volumes.   It also simplifies modifications, although I doubt this is the aim.    European cars generally had a different speedometer for each ratio.      If the ratio is changed, then either the speedometer needs to be changed, or an inline Speedometer ratio box needs to be installed.

As Sydney is infested with speed cameras, I had the cable removed from the car and sent to a speedometer shop.    The original plan was to modify my cable to accept a ratio box.   However, my cable wasn’t able to be modified – it was too thick and the right fittings are no longer made for such a thick cable.    Therefore, a new cable and ratio box was made up.    Having the new cable made up was more expensive than I was hoping, but If I avoid two low range speeding tickets, I am ahead.

Helpfully, the Smiths speedometer actually lists how many revolutions per mile it expects.   In the case of an E-Type with a 3.54 ratio, this is 1312.    For a 3.07 ratio, it is 1120.   These ratios allowed the speedometer shop to correctly build the speedometer ratio box.    For reference, the 3.31 ratio should be 1216 revolutions per mile, and 2.88 is 1058.

The new cable was installed today and a small bracket fabricated to hold the speedometer ratio box.

I have not yet compared the speedometer and odometer to a GPS, but it was reasonably accurate before the differential ratio change, so I expect it to be ok.   This is a timely change as the NSW Government have just announced the speed camera warning signs are to be removed.

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It is possible I have missed a few more country specific ratios.   Please indicate in the comments below if I have.  The data comes from the official technical data books.

If this table is being referred to for swaps, the first and second generations diffs are not interchangeable.   Neither is an ASR diff.   LSD was standard on 560s but optional on other cars.    The exception is cars equipped with ASR that have a different differential.  ASR cars do not have LSD.

In addition, the 8 cylinder cars used a larger case (1.3l) than the smaller engined cars.

My personal view is that with first gear start, the standard ratio could be made taller.   This would improve acceleration and make the mostly useless first gear usable.   This is certainly the case on my 300SE.   I would have preferred the ratio provided for France, but with first gear start.

It is also interesting to see the 560 used a shorter ratio than the 500 or the 420.   It seems clear Mercedes saw it as more of a hot rod whereas the 500 was the autobahn cruiser with its tall 2.24 diff ratio.   In selecting the W126 differential ratios, I’m also not sure why Mercedes Benz went to the effort of producing 2.82 and 2.88 gear sets when they are so close together.

Mercedes-Benz went on a fuel efficiency drive during 1981 which is why the V8 cars got revised ratios.   The project was called the “Energy Program” and included other changes such as the revised Bore/Stroke of the 380 engine.  This project also affected the 107 models.   The 500 received and especially tall ratio (2.24) based on this project.

The other factor that must be taken into consideration is that nearly all W126s were equipped with automatic transmissions.  The Mercedes automatic transmissions of that era did not feature a lock-up torque converter.   This means that actual RPM will be slightly higher than the table indicates due to slip in the torque converter.

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When these cars were new, in America it was important to show good 0-60 times even if this impacted overall drivability.    What the US ratio means is that doing even the slow freeway speeds we have in Australia, the engine is turning at 3500 RPM. I would like to reduce the RPMs at freeway speed, but there are two ways.  Either a 5 Speed transmission, or a rear end change.   The table below shows some of the theoretical options (e.g. The E-Type is not going to hit 274km/h irrespective of gearing):

E-Type Differential Ratios

The most common E-Type differential ratios were 3.54 (USA), 3.31, 3.07, and 2.88.  0.8 or 0.73 5th gears are the most common in the popular transmission kits.

The 2.88 I feel would reduce acceleration too much, and coupled with a 5 speed would be too tall gearing.   3.31 and 3.07 all provide good options.   Without a 5 speed, 3.07 is probably better – with this ratio alone you get much more reasonable drive ratios.  Translating to a more reasonable 2800 RPM at highway speed.  With a 5 speed, the 3.31 is probably more suitable.  Once you have a 5 speed, even the 3.54 can work, although you ‘run out’ of the other gears quite quickly.

From the chart, I would say that if you’re going to use the stock gearbox, 3.07 seems the way to go.  If you consider a 5 speed as well, I would probably go with 3.31.  3.07 as a good option too.   With a 3.31 option, you could also consider the 0.73 5th gear, that would not be as good with the 3.07 and a 0.8 5th gear would be preferable.

I don’t have immediate plans to change my E-Type.  If the transmission failed on me, I would probably install a 5 speed. I may consider a rear end change in the meantime to make the car more useful on the open road.

It should also be noted that tyre size is another factor in determining road speed.   This article assumes the tyre/wheel size staying the same as the differential ratio is changed or a 5 speed installed.   My car does not have standard size tyres fitted so this made the RPM even higher.

Update, November 2016:

My E-Type has now been changed to a 3.07 ratio which I see as a big improvement.  Even around town you can hold the gears longer and use 3rd more as a cruising gear on smaller streets.  I have also fitted a ratio box to correct the speedometer.

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