Have you checked your odometer? Go for a drive on a road with mile markers (i.e. higways), more distance, the better the idea of your odometer error (i.e ten miles based on mile markers, 11 miles based on odometer = the odometer is 10% fast).

I installed a M41 from a 75 240 into my 65 122, and with the stock speedometer gear I was off about 3% fast, which I consider pretty much right on for this vintage of speedometer.

Note: Don't reset the tripometer when the car is moving, the gears in the tripometer will eventually break.

The speedometer and odometer is pretty much independent of each other. The speedometer can be adjusted. The "ribbon" drum of the speedometer is driven by a clock spring, which is tacked in place with paint at the factory (the paint breaking off is the main reason why these speedometer read fast). If the spring is tighten, the speedometer will read slower, and if the spring is loosen, the speedometer will read faster.

This is how I set my speedometer:
-Remove speedometer (real fun, be carefully of the temperature gauge)
-Crack open speedometer (the front is secured by the crome being pinched)
-Remove the screws that covers the speedometer drum (note how everything is put back together)
-You will see the clock spring opposite of where the speedometer cable is screwed in.
-The spring can be tighten or loosen by moving the spring forward or backwards (I use a small screwdriver)
-Get three people/two cars (the other car should have an accurate speedometer-cruise control is a plus)
-two people to drive, one person to set the speedometer.
-leave the speedometer out of the dash, and hook up the speedometer cable.
-go for a drive ( have a signal for a certain speed i.e. wave hand out of the window at 50mph)
-move spring until the speedometer reads correctly (you will need to have the plastic MPH/black metal covers)
-check it a couple of times
-disconnect speedometer cable, and tack the spring down ( I used fingernail polish)
-reassemble and reinstall speedometer

I'll try and post a picture of a speedometer spring later tonight.

Paul