should I switch the differential?

[dropnosky]

I have been reading on some other posts about the 1985 300D diff being a different gear ratio than the earlier models. From those posts it seems that you can go a little faster and get better fuel economy with that diff. My parts car is a 1985 300D, and I am currently planning to work on my 1984 (by the way, title is wrong on this car stating 1983, anyone else have this problem?) 300D rear end, new boots, gear oil change, some brake work, and maybe some shocks. since it would be relatively straight forward for me to do the switch during this work, and when I have the axles out, would it be reccomended?

my ultimate goal is fuel economy while moving pretty fast. what do you think?

and also, since I have the title issue with the 1984, I just want to make sure of my years. where would I find the gear ratio info on the diff just in case it is a 1984 as well?

[AnthroProfessor]

The 1985 most definately has a different gear ratio. I think it is a lot faster and smoother overall since I have compared it to other years my family has.

I would be weary about swaping the differential as I suspect there are other components that also differ on the 1985--the accessories that work with the differential might be specific to a certain dear ratio. You may risk wearing out other parts in your 1984 by opting for a differential it was not designed to take. The 1985 turbo, air filter and perhaps tranny (i have to check) are not the same as the 1983 or 1984 model.

Keep in mind that these cars are overengineered and modifications generally do not do well with them. Even adding something simple as a K&N air filter does not make the engine breathe better since the limiting factor on the car is somewhere else--I think the turbo does not allow anything excessive.

As far as your title issue and knowing what gear ratio your car has..... How sure are you that the car chasis has the same engine, tranny and differential from day1? I think the differential has a unique serial number on it that is matched to your chasis. Call the Merecedes Classic Center in Irvine and ask for the data sheet so you can see what equipment your car came with. I know you live in Missouri, but according to the website, they will pull out the data sheet as a courtesy to MB owners. I would start there as I am sure you will get to the bottom of what model year it actually is and what differential it is suppose to have.

[Maxbumpo]

You need to purchase the Technical Data Book for passenger cars, 1985 April edition, from MBUSA. This little gem will have all the info you desire, on all MB cars made between 1983 and 1985. Gear ratios, torgue specs, dimensions, capacities, enough numbers to keep you occupied for days and days.

An experienced MB mechanic can generally repair most things on the car with only this little book.

I wonder if the gear ratios in the transmissions are different? Also, speedometer gears will be different to need to swap those parts as well.

Renew all the rubber bits you can while you have it apart (flex discs, diff. mounts, rear subframe bushings, etc.)

[dropnosky]

thanks, thats some good info, I will look into it.

the truth is that I have no idea if the 85 has all original equipment, but it is an 85 according to the doorplate.

[JPMose]

How could a 1985 be faster? The gear ratio is lower....so you have a lower RPM at highway speeds, which is nice....BUT....wouldn't that equate to slower off the line?

[dropnosky]

I meant faster top speed, sorry, I should have been clearer. I am not as concerened with the acceleration as I am with cruising speeds on the highway