That makes sense, since the change in the gear ratio is 2.73/3.55 = .77 which is almost exactly what you got comparing your speedo reading to your friend's.
Unless someone here knows what the correct speedo gear for an M20 with a 2.73 axle is you are still going to have to pull the gear and count teeth. Multiply the number of teeth that you have times .77 to get the number of teeth that you need. For example if you have 26 teeth now then you will need 20. A speedo gear only costs about $5 so if you are still off by a bit you can increase/decrease by 1 tooth or so without spending a bunch of money. I've done this just to reduce error caused by putting big tires on a car.
Its been a long while since I had a Muncie so I don't remember the range of teeth available. Also I think GM may have used different speedo drive gears in the trans for different rear end ranges, which may limit you without going into the trans or buying a ratio adapter. But my bet is that all you will need is a new speedo gear.
Walt
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1969 Lemans vert, matador red, 462 CI, 3.07 12-bolt posi
1974 455 TA, admiralty blue/red interior HPP "cover car" - sold
"The best way to show a car is to drive it"
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