It depends if the car is a CA model with the 722.4 or Fed model with 722.3 with 1.48 or 1.44 3rd ratio
I have two 2.88 speedos, one with three dots at 74mph other at 77.

Good point but the 722.4 was only installed on CA models and only in 1985 so it's a rare speedo but at least we know that it has 3 dots at 74

To be clear: My above referenced "1985 300D" is a CA model and indeed, has the factory installed 722.4XY.

I know this sounds weird, but the speedometer, trip odometer and the odometer are all separate instruments, geared off the one speedometer cable. If you've verified that the trip and regular odo's have the correct reading, then the instrument is correct for the car, and well matched to the tires and diff. That means that the only problem is that the speedometer is reading low. That can be easily adjusted by pulling the needle and repositioning it slightly. Before you begin, use a short length of speedometer cable to drive the speedometer with an electric drill. Not the speed reading. Then just adjust the needle so that it reads 5mph faster.