I need another gear or a taller one!!
 

I have an 83 300SD and it turns 3K RPM at 65 MPH. I sure wish I could slow the engine down a little. Is that feasable??

Aside from fairly major swaps/changes, that is just the way it is. The engine needs to be at least around 2700 at 70 in order to have enough power to get up hills ok.

That is normal. The engine needs to be in that power band at speed to have any kind of acceleration.

You could swap a 2:88 in there but you will notice little difference and it is a lot of work for very little drop in RPM's. There are folks that have swapped in a 2:47. I would not do it. But, "there are folks that have done it". FYI....

A search will bring enough reading to keep you busy for days.:rolleyes:

We swapped a 2.47 into my dad's 300SD.....it does lower the RPM's but acceleration suffers. If the car is used on mostly highway for commuting/cruising it is a good conversion, but a fair amount of work and $$ to get done right.

What did a 2.47 come out of?

The rear diff swap has been discussed on here many times before. I did the swap on my 1984 300SD and spent about $150 (including Mobil 1 gear oil) to do it.

If you are looking to slow the engine down on the highway the 2.47 will do it, but it will also slow it down in town as well. Your acceleration will be slower.

With regards to fuel economy, I religiously tracked every tank of gas before and after the conversion. The improvement was slight, about 1 mpg if I remember correctly. The lower engine speed was offset by the need to hold the pedal down further to maintain the same speed. I am in the flat midwest, so if you live in a hilly area I expect the economy would be worse.

If I had to do it over again, I don't think I would. The car is fine running at 3k-3.5k and will do so happily all day.

As I said, this has been discussed numerous times. Search the Diesel Discussion for "2.47 diff" and read away.

I never really looked that close. But, I looked at it on the way home.

In mine... An indicated 65 MPH shows 2800 RPM. That is with the stock size tires (no plus sizes) and the 2:88 diff.