Just picked up a 1980 300SD. Great car! Got it cheap, it gets decent mpg, fun/easy to work on, feels like im riding on a cloud its so smooth etc etc. I drive around 40 miles one way to work and its allll highway. Im pushing 3000 rpms at 68 mph, I want to find a diff with a better gear ratio for highway driving that will drop into my car. I guess if I could find just the gears that would be cool too, but a direct bolt in diff from a gas car would be better. I've read about(forgive my ignorance if im wrong on this, Im new), 2.67, and 2.47 gear ratio's in the gasser cars of the same era as mine. If anyone has anything available or knows a good place to pick one up I'd appreciate it. I live in Michigan.
Thanks in advance

The 450SEL of that era has a 2:65 ratio but it is a good bit wider and would not fit with your axle shafts. All of the V8 diffs are wider. Most of the other gas cars had a taller rear than the SD which is 3:07. Maybe someone else has done a swap and could shed some light on it. I assume you know that if you swap the rear end, your speedometer will be off.

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Cars:
1993 400E
1993 300TE Wagon
1985 300D Turbodiesel w/ 688,000 Miles On Original Eng/Trans
1979 300SD
1977 Euro 6.9
1967 230S IMA Universal Wagon
100 Or So Parts Cars

You Cannot Change your Future Without Disturbing Your Present

It Looks A Whole Lot More Like It Did Today Than It Did Yesterday?

3000rpms is just fine, just take some time to get used to it - you were just used to a totally different type of engine before.

I believe early model 126 diffs can sometimes interchange. I've got a 3.92 diff if you want, lol...

Does your cruise control work?

Driving in my 450SE as I post this. 3000 RPM's at 65mph, and I can do 130 in the same gear. What's the problem?

LOL.

Would I improve my gas mileage by swapping though? Im not really concerned with performance. As far as the speedo goes that things been broke for a while so maybe I can swipe the speedo cable from the donor car too.

Just curious, if your speedometer is not working, how do you know that you are turning 3000 at 68 mph. You must have a GPS. I know one thing, our Dodge Ram Cummins Turbodiesel gets horrible GAS mileage. It only made it one mile on a tank of GAS. The fuel mileage on DIESEL is not that great either. I really don't think your fuel mileage would improve that much with a slightly lower gear ratio rear end. It would be very difficult to figure it anyway without an accurate speedometer. I think the SD's get pretty good fuel mileage to begin with for a car of that era.

__________________
Cars:
1993 400E
1993 300TE Wagon
1985 300D Turbodiesel w/ 688,000 Miles On Original Eng/Trans
1979 300SD
1977 Euro 6.9
1967 230S IMA Universal Wagon
100 Or So Parts Cars

You Cannot Change your Future Without Disturbing Your Present

It Looks A Whole Lot More Like It Did Today Than It Did Yesterday?

Yeah I got the mph from a Garmin gps. I dont seem to be getting that bad of gas mileage but I drive all highway to work and would like to be able to cruise at 80ish mph.

So cruise at 80. It's no problem at all with the stock diff. Heck, I routinely cruised at 80 with a 3.92 diff in the 200D, which had much higher RPM's at that speed.

I think you're probably just not accustomed to the sound of the engine.

Doubt you will increase efficiency much by changing the diff, maybe a little but depends on your overall driving. These cars aren't bad on mileage at all, getting 25-35mpg. My 450SE (current daily driver) gets around 12mpg.