3.92 to 3.69 RE ratio in 69 with Auto?

[hank sound]

Hi folks,

My Auto equipped 69 280 (Ingrid) has a 3.92 rear end ratio.   On the highways of So. Cal., RPMs are up there when at speed.    Aside from the "original numbers issue", what are your thoughts regarding a change to a 3.69 ratio rear end?    If anyone has done it, were any negatives observed with this combination of 2.8/Auto/3.69 setup? - - My first thought that prompted this question, was how driving in the hills would be affected, and how a 3.69 RE ratio would change the MPH speeds at which the Auto would shift (requiring too high a road speed).     I know the engineers usually figure this stuff out but since I see it as only a .23 difference in ratio, and honestly don't know if that's a little or a lot, I guess it doesn't hurt to ask

Respectfully,

Hank

[Larry & Norma]

My 2.8 auto had 3.69 from new. Drives fine. From memory about 3500 rpm at 70 mph

[Benz Dr.]

Frive speeds cars are 4.80 for the most part and that's what my car had. I changed it to a LSD with a 3.92. That's only 160 RPM less but I can tell the diffrence in first gear. I often started in second gear with the 4.08 but the 3.92 seems well suited to the 5 speed gear ratios.
 Road speed is just a bit less trottle to go the same speed and I've picked up a bit of mielage with the taller gears. If you change from 3.92 to 3.69 you will lower RPM by 230 which is only 70 RPM more than what I did. You will notice a diffrence with anything over 200 RPM.
Standard axle ratio in most 230SL's is 3.75 which is a bit too high for that car. 3.92 would work better. The best ratios for each model would be 3.92 for 230SL, 3.92 for 250SL, and 3.75 or 3.69 for 280SL

[jameshoward]

Frive speeds cars are 4.80 for the most part and that's what my car had. I changed it to a LSD with a 3.92. That's only 160 RPM less but I can tell the diffrence in first gear. I often started in second gear with the 4.08 but the 3.92 seems well suited to the 5 speed gear ratios.
 Road speed is just a bit less trottle to go the same speed and I've picked up a bit of mielage with the taller gears. If you change from 3.92 to 3.69 you will lower RPM by 230 which is only 70 RPM more than what I did. You will notice a diffrence with anything over 200 RPM.
Standard axle ratio in most 230SL's is 3.75 which is a bit too high for that car. 3.92 would work better. The best ratios for each model would be 3.92 for 230SL, 3.92 for 250SL, and 3.75 or 3.69 for 280SL

Just in the process of rebuilding a 3,46 (with help/direction/knowledge of Naj, of course) to put on the 230 to replace a 4,08. I've been umm-ing and ah-ing about the change since I was told by a respected classic MB garage in the S of the UK that a 3,46 was going to slow (nippy-ness) the car significantly. However, because most of my miles tend to be on long hauls, I think I'm happy with the loss of nippy-ness, in order to get the lower RPM at higher speeds, spare the engine (low compression) and save fuel. A few here have gone down the 3,46 route (David B, I think) and have made favourable observations. That said, at the last Euro Event, the short box was excellent in the hills and sharp turns - it just kept on pulling, and far better than many modern cars.

I think I'm reconciled to the change, but I think I'll keep the old diff just in case. But interesting to hear your thoughts, Dan. I'll bear it in mind. (4 speed manual, late 230).

JH

[Garry]

When I had a 4 speed gear box fitted with the 4.08 ratio i did not like the higher revs at highway speed and as most of my driving was highway I decided to change it. 

I installed a 3.46 and the effect on driving was very noticeable with the lowering of the final drive reducing RPM by around 300.  It made the pleasure factor for highway driving just that much more.  I did not notice any great change in acceleration that was of any concern.

Since then I have installed the 5 speed in the car as well with out putting back my old 4.08 axle.  Now there is a huge change in the revs.  4th appears to be the same as a four speed with the 3.46 but when put into 5th the revs drop down to as low as 2200 at 60mph so it is strictly an overdrive.  When driving around hilly roads I just use the gears more in the 1,2 3 range and find it fantastic.  Fuel consumption improvement is very noticeable.

My only question is should i change back the rear axle to the 4.08.  At this point I wont but I do have to keep reminding myself to drop down a couple of gears if I am passing. I am sure those that like spirited driving all the time would not like it but as a highway cruiser, it is fantastic.

Garry

[Benz Dr.]

Better check the math. 4.08 to 3.46 is more in the range of 600 not 300. You would definately notice something in that case. With the 5 speed you will lower it another 500 RPM's which might be too much.
The 5 speed was given the 4.08 axle so that it would pull really hard in the lower gears yet still be nice in 5th gear. The 4.08 in 5th gear is the very same RPM as your 3.46 with a 4 speed box.

[Garry]

Dan,

You of course are right it lowered the revs to around 2700rpm with just the axle change but still the 4 speed and then down to 2200rpm with the 5th gear for 100 kph.

I ran out of fingers and toes, now I know why I wasn't an accountant.

[jameshoward]

Garry,

There's the maths, and then there's judgement. So I'd say it depends on what sort of driving you're doing. If it's flat, long haul stuff, you should be OK with the taller gearing, but if you plan to head off to the Alps (or the Aussie equivalent), or your driving is more stop go, you may feel it better to change back. But please note, I (we) haven't got around to making the changes on my car yet, so my thoughts are hardly worthy of consideration. I might perhaps feel that a 5 speed box and a 3,46 may be somewhat excessive.

Can't say I wouldn't mind being in that particular fix, however. So, I recommend you send me your 5 speed box, and that way you won't have to change the axle again. Sound fair?

James

[Benz Dr.]

3.46 is a good final drive ratio. Low enough that you won't have to gear down to pass a car yet high enough to reduce engine RPM's and noise. 

[Garry]

Given the tallest mountain in Australia is 2,228 meters and 'the big hill' near where I live is 1000 meters above sea level but only about 300 meters from the surrounding countryside, I did go for a drive yesterday and went up the big Mountain all of 300 meters.  I can drive from my property to the city, appx 80 km (50 miles) with out having to change gear so cruising is the order of the day

But I do agree that the 3.46 with 5 speed is an over kill and at some point when I put the new ZF in, I will change it back to my original 4.08 axle.  

Now James I have a very good Getrag Gearbox and conversion kit that will become available .....(and a 3.46 axle) i have to recover some money to pay for the ZF factory new box.

[hank sound]

Hi all,

Reading thru the replies so far, "gnuface" seems to feel that the 3.69 would work just fine for my 280 with Auto.   I don't have a four speed or a five speed (yet) and the additional information many of you have posted here regarding those two trannys will thankfully, be here for me if it becomes applicable.   Does anyone else have information to share with me regarding rear end ratios in a US 280 with Auto?   

To everyone, thanks for the input.

Cheers, Hank

[Benz Dr.]

Almost all of the 280SL's I've seen have been 3.92 which seems to be a standard ratio. I've seen the odd 4.08 with 3.69 being very rare. I've seen more LSD than 3.69 axles and the LSD is very rare.

[Larry & Norma]

I think I read somewhere that the 3.69 option was not offered in the American market as
the guys at Mercedes decided you would prefer acceleration to economy as your fuel
was so cheap in those far off days.

[hank sound]

I think I read somewhere that the 3.69 option was not offered in the American market as
the guys at Mercedes decided you would prefer acceleration to economy as your fuel
was so cheap in those far off days.

I believe that would be about right

[Cees Klumper]

Hi Hank - I did the swap on my 69 280 SL auto (so your car) - I went from the 4.08 standard US issue that mine came with to a 3.69. Made a really nice difference all around - accelleration did suffer a bit but it's still quite acceptable, and at cruising speeds the difference is really noticeable. So I can recommend the swap based on my experience, although the impact in individual cases will depend also on the (state of tune of the) engine.

[Peter h]

I also have 2 years ago changed from 4:08 to 3.46  It was a good decision. My 280er engine was completely overhauled before and so i have no problems in the mountains.
 For large gradients i can switch to the 3 speed (auto) and on the flat I spare the engine at higher speeds
But better is a 5-speed transmission with 3.92, no question. With the 3:46 and 4 speed, the individual gears are a bit long, sometimes

 Peter

KM/H conversion table from the pagodentreff

[Erik]

Hello,
 
I have a 230SL automatic with the 4.08 ratio.
I want to reduce the highway RPM’s and, most importantly, the fuel consumption so I plan to switch the ratio from 4.08 to … ?
I have a choice here, at the same price, to fit the 3,69 or the 3,46.
I seldom use my car in the mountains but where I live (traffic lights, speed bumps,…) I have to stop and go a lot.
My engine is under rebuilt and will be fitted with the high pressure pistons and a modern state of the art fuel injection system. So after the rebuilt it should benefit from the power and torque as if it was factory-fresh.
What is to your experience my best option, given the fact that the automatics have that strange habit to start in 2nd instead of 1st ?

Thank you for any advise,
Greetings,
Erik

[Benz Dr.]

I also have 2 years ago changed from 4:08 to 3.46  It was a good decision. My 280er engine was completely overhauled before and so i have no problems in the mountains.
 For large gradients i can switch to the 3 speed (auto) and on the flat I spare the engine at higher speeds
But better is a 5-speed transmission with 3.92, no question. With the 3:46 and 4 speed, the individual gears are a bit long, sometimes

 Peter

KM/H conversion table from the pagodentreff

That's what I have right now - 3.92 with the five speed. I think the combination is just right.

[hank sound]

Hi Hank - I did the swap on my 69 280 SL auto (so your car) - I went from the 4.08 standard US issue that mine came with to a 3.69. Made a really nice difference all around - accelleration did suffer a bit but it's still quite acceptable, and at cruising speeds the difference is really noticeable. So I can recommend the swap based on my experience, although the impact in individual cases will depend also on the (state of tune of the) engine.

Hi Cees,

More and more, I feel that a change to a 3.69 ratio for my 69 280 with auto will be the best choice for my future upgrade.   

Thanks all, for the feedback.

Cheers, Hank

[Peter h]

Hi Erik

230er with 3.69 This will fit. My Opinion
Peter

[hank sound]

HAPPY HOLIDAYS !

Today, the truth (mine, at least) was revealed...... A friend of mine who has a 70 280SL with Auto, let me drive his 3.27 rear end ratio equipped car    Knowing full well that my 69, if comparably equipped, would never be involved in a drag race, I drove his car as I would drive mine on the streets and freeways of the greater Los Angeles area.   Hell, one can even get a bit aggressive with the Auto shift lever, if necessary. 
 
Green light launches take place in a split second................. but driving on the freeway, can last for hours.    It's a 3.27 for me !!

Happy Holidays,

Hank