Speed vs. RPM '85 300D (in lieu of repair)

[FrankinAZ]

Good morning .....

I'm the proud new owner of an '85 300D (USA model). I bought it specifically for an 8000 mile, around the states trip. But now, after driving the car for the last several weeks I'm falling in love with it and can see keeping this car for many years to come. It's built to such high standards ... just amazing.
Anyway, my area of concern is a pessimistic speedometer. She reads slower than actual speed but operation is smooth as silk and consistant. No noise.
Does anyone have any rpm vs, speed data for this car in top gear (4 speed auto)?
A second concern ... the transmission was rebuilt (by a well known local German auto garage here in Phoenix) 20k miles ago and shifts/performs flawlessly) but I'm concerned that it may have been an off the shelf exchange and has a wrong speedo drive gear in it ... is that possible?
The data will be fine if someone has it and would be greatly appreciated.

Thank you