George/Sil,
As far as I have observed (very early model 240s with the same speedo as the 73-74 140s) during my rounds at Pull-A-Part, the cars that came with the auto box had the speedometers marked 0,99 and the ones with the stick had the 1,03 ratio. Then again, it could have been coincidence.
On my 74 144 with the m-40, I replace the non functioning speedo (1.03) with one from a 77 244 with the auto tranny (0.99). It is pessimistic by about 5 mph. According to my 74 144 brochure, at 1000rpms in 4th gear, the car should be moving at 19 mph (your gear ratios might differ unless your M-41 came out of a 74). Do the math, and you should come up with the answer, provided you have an accurate tach. My factory tach shows 3000rpms at 50mph, with respect to the 0.99 speedo, (actual speed 55). I found out the error on my 144 few months ago while driving past a speed detector which displays your speed on an overhead signboard as you pass by.
Like George said, try swapping speedo with different ratios around. My guess is that the one with the 1.03..er I mean 1,03 ratio should be "purdy" accurate, given your M-41.
When you are done with the speedo, you might want to start swapping the gas gauge around...

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...and the bricks keep on rolling