Hi guys...

I drive 80mph on I-25 near Albuquerque every day commuting...my '85 300DT does, what, 3400rpm at 80mph?

I'd REALLY like to lower those RPM's...I've read a couple threads here, but I haven't seen a reference to:

1) exactly what rear end part I need and where and can find it, and
2) how much $$ that part is, and
2) about how long/hard it is to do the swap, or
3) about what a Mercedes mechanic would charge to do the job.

I'm looking for pretty much lowest possible RPM's, but I'd go a little higher if that prooved beneficial in some way.

Thanks...

Type Easter Egg in your search box and you will get a ton of info on gear ratios.

__________________
1983 300-D turbo
1985 300-D turbo
1959 Harley Panhead chopper
1929 Ford coupe restored
I hang out with Boneheaddoctor at Schuman Automotive OBK#5
All liberals are mattoids but not all mattoids are liberal.

Thanks J.R.B., those posts were enlightening to some extent, but I couldn't find answers to my questions.

your car i believe has the 2.88 gear ratio in it. other cars of the same age such as the 380 s have a 2.65 gear. you will need such a rear end and the speedo to go with it. i estimate you can find the rear end for 2 to 300 and the sepedo for from 50 to 100 exchange. the benz store has both available, at least they had them when i needed them. not this specific rear end but others. the speedo is easy to change. you can do it yourself in an hour or less. unless you have a lot of tools etc you may want to farm out the diff change. i estimate that it will cost 3 to 400 labor done by an indy. this change will lower your engine speed and increase your mileage by 5 to 10%, my estimate not from personal experience but from what others have posted about similar changes. this change will make you r take off from stop a little less speedy.

__________________
[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.

I have seen all the exact info you are looking for posted here recently. Lots of info.
It is worth the effort as your search will surely benefit you with the info you seek.

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

Is this the math for the rpm change?:

2.65/2.88 x given rpm = new rpm?

So:

.92 x 3400rpm (at 80mph) = 3128 ?

While that change would be welcome, I'm looking for a lot more than that...I'd like to get down to maybe 2500 rpm at 80 mph.

I don't know much about these things, but my 300DT revs seem high for 80mph...esp for a large car which typically is designed to loaf...seems to me, just intuitiuvely, that the rear end would be higher than 2.88.

What about the 2.47 that I've read about? Is that available for my car? That would potentially give me 2915...hmmm...nuts...again, welcome, but not the change I'd like to see.

Thanks...

I don't think the inductive speedo of a 380SE/L or SEC will work with a cable drive speedo of a 300D. A 380SL/C might have a cable drive speedo. A 1980 450SEL should have a 2.65 and cable drive speedo.

Nitske says some Euro 500 SL/Cs, SE/Ls and SECs have 2.24 rears. There's a fair chance of finding these gray market cars in wrecking yards now.

There's also a 5-speed conversion whether a 5-speed MT (0.78-0.80) or a 722.5 (0.75).

I don't know how hilly Albuquerque is. An older tired turbo might not be quick enough to answer the call of a hill at 2500 rpm.

Sixto
95 S420
87 300SDL

We live at 5700 feet, with plenty of high mountains, though the interstates are pretty flat...I don't mind if the car is a little slower, I just don't like listening to the engine drone all day at 3400 rpm...lots of noise and it just seems like it's working so hard.

I don't think you'll ever be able to get those RPM's down to that, even with a 5 speed. Our 300TD's (as well as 300D's) weren't made for that type of operation. I think if you get 2915 out of it, you'll be happy. Its just you can't lung these cars down, they're made for that "high" speed operation, just ask anyone. BTW, what condition is your hood-pad in?????

__________________
-Randy Wakefield

1969 220D 4 speed (parts car)
1976 280C
1976 280S
1981 240D 4 speed (parts car)
1982 300D
1983 300TD (Ivory)
1983 300TD (gold)
1985 300TD (gray)
1987 190D 2.5
1970 280SEL (sold)
1977 240D 4 speed (sold)
1974 280 (rusted to death)
---
1927 Chevy 4 door sedan
1938 Chevy 2 door sedan
1950 Willys Jeepster
1955 Studebaker President 4 door
1977 Ford F250
1979 Glastron 17'
1948 John Deere A
1960 John Deere 2010
1979 Satoh S650G
--
2000 Bichon (Doby)

first, i think that the 85 300dt has the electronic speedo referred to above.
the 2.47 rear end was installed in a 300td i believe, i am not sure what year. the installation did require some machine work. if i lived at 5700 feet i dont think i would consider the 2.47.

the math looks ok.

a manual conversion to the 5 speed would be nice. i have not heard of anybody putting the 5 speed from the 190s and 300s (E) behind a 617 turbo. i would be concerned of its strength. I am not saying it is too weak just that i dont know an would need more info.

the 5 speed that came in the 123 bodies and some sls is very robust and would be plenty strong, i think.

for a manual conversion you will need pedal assembly, tranny, shifter, flywheel, cutch assembly, and the driveshaft would need to be adjusted for length (probably). as a trade off you could avoid changing the rear end. the 5th gear ratio in the older style 5 speed is .813 so take that times your rpm at a given speed to get the resulting rpm if a five speed were used with the same rear end.

for me a five manual is the ultimate, unless i could figure a way to jump right to a six speed. i noted on the performance section some guys contemplating a six speed conversion kit but the cost was in the 7-10,000 range i think, so this would be a very long payback.

the older 123 five speeds go for i believe 2-4,000 used... and are rare as hens teeth, perhaps rarer.

__________________
[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.

Pretty good...intact, but a bit dry...I'm pretty sure it's original...I'm the third owner and I have ALL the maintenance records since it was new...I'll have to check, but I think it's original...why do you ask?

And yeah, 2915 would be a welcome change, I just wanna make sure that's the best rpm, all things considered.

Mine has a cable speedo.

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

are you sure your car is not a late selling 84 model? it is my understanding that the 2.88 rear end cars had an electronic speedo. is your car equipped with the 2.88?

__________________
[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.

Looking into rebuilding/replacing my slipping, and when hot non-functional 722.118 4spd in my '81 300D soon, but would really love to hear more about a 722.5 conversion!
It would be a godsend to lower the R's at calif highway speeds (70-75mph), without making this beast any slower off the line!

David Howard
david_howard@yahoo.com

I asked that because you said the car sounded a bit "loud" or something at highway speed, and a hood pad in good condition will reduce a lot of noise. I have driven these cars both ways and the noise level is night and day.

__________________
-Randy Wakefield

1969 220D 4 speed (parts car)
1976 280C
1976 280S
1981 240D 4 speed (parts car)
1982 300D
1983 300TD (Ivory)
1983 300TD (gold)
1985 300TD (gray)
1987 190D 2.5
1970 280SEL (sold)
1977 240D 4 speed (sold)
1974 280 (rusted to death)
---
1927 Chevy 4 door sedan
1938 Chevy 2 door sedan
1950 Willys Jeepster
1955 Studebaker President 4 door
1977 Ford F250
1979 Glastron 17'
1948 John Deere A
1960 John Deere 2010
1979 Satoh S650G
--
2000 Bichon (Doby)