I still like to be able to climb significant hills at highway speeds (75-80 mph), and I don't believe I could do that with a 2.47 and a 617 turbo. Also, where would I get the correct speedometer for a W123 with a 2.47 calibration?

Sounds like you have a job kind of like mine.:D

Refreshments are important.:D

Well, my 300na is fine with a 307. I cannot imagine a 300d turbo struggling with a 247. Hey, that is what gears are for.

I don't have an answer for the speedo though.

I think maybe a sticker on the speedo at the desired corrected cruising speed might be enough.

AT $5 a gallon sacrifices must be made!

Tom W

I don't really care about the fuel mileage, I'm really just trying to minimize the engine wear and the noise by running slightly lower rpms. It would be nice to reduce the rpms a little, but I'm not willing to sacrifice any drivability. A 2.88 would be convenient because it bolts right in and I could just order the correct speedometer. If it takes much more effort that that, I'm not interested in changing it. I was thinking about having my indy toss in an '85 differential/speedometer while he's doing the engine, but I think I'll do one thing at a time (just in case I need to do some troubleshooting).

I'd go for the 85 diff. Thats one of the reasons I would hunt for an 85 if I was W123 shopping. The geared that year a bit better to keep the RPM's down on the highway.

2.88 rear end ration

I probably will put in an '85 differential eventually, I just want to make sure this engine swap is OK first. I would hate to change more than one thing and end up with a noise or vibration and not know why.

I don't think I would buy an entire '85 car, there are to many extra little complexities that I don't need unique to that year.

Thats like 3%....why bother?

If your rear end was toast then that is the only way such a swap would make any sense to me.

Tom W

More like 6.6%, that would reduce my "normal" cruising rpm from about 3500 to about 3300 rpm. It would make it a tad quieter and it might improve my mileage by 3%, maybe 1 mpg if I'm lucky. I'm not in any hurry to do this, if I really wanted to improve my fuel mileage I would just drive a little slower.

A 2.47 would be a 24% change, that would correspond to about 2800 rpm at 75 mph. I'm not sure that engine would develop enough power at 2800 to climb significant hills at 75 mph (I'm sometimes near the limit with my 3.07). I don't think I'm willing to deal with that much of a performance compromise.

no problems with 2:47, I did it and wish I had done it when I got the car,the difference in performance is slight and the mpg gain is about 15-20% on the highway, you won't regret it.

Not at 6000 feet above sea level with 7% grades on the interstate and 75 mph speed limits. There are times I wished my gearing was a little lower, I'm actually afraid I will regret going to 2.88 (if I ever get around to trying it). I don't care about the fuel mileage.

ok:
for the ignorant people here; In what cars you find the differentials you guys are talking about?

This catalog will give you a pretty good reference of which ratio was in which car:

http://www.adsitco.com/category.asp?CID=50

Help! Help!
I do own a 1969 220D MT I am the 5th owner. The Car is a mess some times it will not go into gear but when it drives it drives very well. I love that W115 chassic very much. It have over 355,700 miles on it witch speedometer dosen't work. However, I took it for repair at Auhaus Benz in Las Vegas today. I will ready like to do a complete over haul. keep my W115 chassic and put a rebuild bulet proof W123 1979 300SD automatic engine, transmission, drive shaft train (RWD) in it. Has anyone done it before and can I do it?
thanks DPL