Speedometer question

[Yooper truck]

Last weekend I swapped out my speedometer with another factory speedometer. It had been making grinding noises behind the gauge cluster and I had already changed the speedometer cable.

The newly swapped in speedometer took care of the grinding noise, but now is off by about 5 mpg. It is reading 50 mph when I am going 55 mph which I confirmed by GPS and timing between miles. It was just about dead on before the former speedometer started acting up.

I have looked up several posts and everyone references the nylon gear on the transmission end to change the speedometer reading. I didn't change that, all I did was took off the old cable and put on a new one and then swapped out the speedometer.

The new (used) speedometer came out of a 1972 four wheel drive suburban and I have a 1971 2 wheel drive. Is there a difference on the speedometer end between 2 and 4 wheel drive?

Anybody have any thoughts on this?