Hello all, I thought I would take a few minutes to write up my experience with this over the last few thousand miles. I found little to no info and lots of speculation trying to research before going forward with it.

Car is a 1980 240D all stock with 4 speed manual trans. Original ratio was 3.69. I am running 195/70R14 tires which are about 24.8” tall. Car is in good tune with adjusted valves and timing bumped 2-3 degrees.

I use this car to commute on the highway 70-80 miles round trip everyday. I am in Connecticut which I would say is moderately hilly. Very little of my drive is flat, but there are certainly no long steep grades to worry about.

Initially, I tried a 3.46 diff which I would not describe as much more than noticeable. It felt and sounded a hair happier at 70mph, but not much else.

I started searching for a 3.07, thinking a 3.27 would be perfect but would be like finding a needle in a haystack. A local guy I know had a 2.88 already out of the car for $50. 3.07 was a 13% decrease from 3.46 and 2.88 was 18%. Meh. After tackling the first diff swap I felt like I had a good handle on it and it was worth the $50 and 1.5hr experiment.

Somewhat unorganized thoughts/impressions:

By myself doing 70-80 with left lane traffic it works really pretty well. There are a couple hills that have me foot on the floor losing speed if I didn’t hit it at 75-80, but honestly... that’s not hugely different from how it used to be. Starting off requires a little clutch work to get going, and takes a slippage to get moving if starting going up a hill. Once you’re moving, it doesn’t feel much slower than before. Low HP motor with good torque and flat torque curve. I merge every morning on a long uphill on ramp and I am able to use the torque from 40 to 60mph in 3rd gear which actually feels easier than trying to make it rev higher than it really wanted to in 3rd and bogging and merging in 4th like it with the 3.69. 4th gear is not used for anything other than flat ground below 50mph and 3rd still feels good up to 50-55 for cruising if needed, not winding out. Having a second person and their stuff in the car does slow it down a wee bit.

I am getting right about 30mpg doing 70-80mph. About the same mileage I was getting doing 65-70 with the 3.46 diff. For my long commute, I really enjoy traveling at faster speeds without losing any fuel economy.

Overall, I think the 3.07 would be ideal. I may very well swap one in at some point. But, I haven’t been able to find one easily and the 2.88 is really pretty good. It has not caused the car to be undriveable or caused any of the other catastrophic things I read lots of people speculating would happen, at least not in the last ~3000 miles. I much prefer it for MY driving over the 3.69 or 3.46, YMMV.