Something doesn't make sense with the numbers. You shouldn't have a consistent mph difference across a 40 mph speed change - assuming you're in 4th gear the whole time. I would expect the absolute error to change with speed but maintain a consistent percentage difference. Going from a 3.46 to a 3.07, I'd think about 12%. Non-OE size tires will change that.

If you're trying to figure out if there's a speedo that you can use that will get you closer, you'll probably need to figure the different ratios between tire sizes + the tranny + the diff.

This site appears to have the gear ratios and final drives: W123-300D-Getriebe

You're using a TD rear end @ 3.07, but maybe 300D 4 speed gearing @ 3.9, 2.3, 1.41, 1.0?