Mark I am thinking of doing just that. I'll need tires in the next few months.
As far as fuel mileage, I have filled twice since the change and one was only a top off after 100 miles. It was just over 26 mpg but the long trip (over 400 miles driving 65-70) was 28.8. Average so far this winter has been in the 25 range. I fill up about every 5 days so I'll keep track and post anything noteworthy.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".

FWIW, it may not be that big a problem today, but sometimes tires from different manufacturers won't be exactly the same size, even if they're marked the same.
My latest 240D came with a set of 8-year-old, dry-rotted 205/70R14 Firestones mounted on Buntd-rims. They rode well, but looked huge on the car and more suitable for off-roading. And they cornered like marshmallows!
I had a spare set of Buntd-rims with old 205/70R14 Michelin-Xs. The Michelins were nearly 1.5 inches smaller OD than the Firestones.
The new 195/70R14 Michelin-Xs I put on the car were very close in OD to those old 205/70R14 Michelins and my odometer accuracy appears nearly dead-on. Plus steering response is much better now.

Happy Motoring, Mark

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DrDKW

Well, after another couple hundred miles I'm still happy with the change. I've tried holding it in 3rd til 60 mph and it makes a difference but it really sounds wound tight at that speed. 50-55 sounds less tortured and works out ok for acceleration purposes. I did a 0-60 time and it has dropped from 28 sec to about 24. That's leaving it in 3rd til 60 which seems to result in better acceleration.
To any of you who are thinking about making a final drive ratio change on one of these, I say do it. If you have a lot of highway driving with low to moderate hills you won't be disappointed and if you have a lot of in-town driving the first 3 gears are more useful also. The cabin noise level is soooo much better and the driveability (except for uphill between 50 and 60) is greatly improved. I still need to make the best of the state-of-tune of the engine but even without a fresh valve adjust and timing check the overall effect is positive.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".

I have slowly come to the additional conclusion that all lift pump pressure determing springs should be changed out on the 616 engines.The 617 lift pump springs are easily obtained used.

The availability of higher lift pump pressure somewhat minimizes the effect of poor flowing fuel filters as the accumulate junk from the fuel slowly restricts them. First thing to occur is the relief valve no longer is remaining open as you are then below the threshold point.

This alone changes the injection pumps ability to function correctly to some degree. So without a pressure gauge installed the higher available pressure from the 617 spring is going to protect that number one rod bearing somewhat better than the 616 spring just in itself.

Those pressure spikes do not see either the return line or cigar hose when the relief valve is closed. As operational pressure declines the elements for some reason indicate they no longer load equally.

I really believe that is why the number one rod and cylinder wear excessivly on the 616 compared to the other cylinders. There is really no way to verify this thought other than setting up and doing certain tests that I really do not have time to do.

Just on the chance I am right about this it is really wrong to ignore the possibility. Nothing has occured to make me change my mind since I first considered it the probable cause.At least of of the number one rod bearing failures.

There is a much lesser effect on the 617 but the same theory applies. It is not only attributable to fuel pressure. Possibly the use of higher lift pump pressure to service the system on the 617 may account for a good portion of the difference as well.

I am still waiting to see a lot of initial pressure reports from people using pressure gauges to get some sort of handle on how widspread low pressure at operational speeds and others is present. I believe the 616 engine may even be a little less noisy at speed with good operational fuel pressure versus lower. I still also feel the installation of a gauge is a good ideal.