Here's an interesting writeup I tripped over by Phil Singher on checking speedometer accuracy
http://www.vclassics.com/archive/speedotip.htm

You need a fairly accurate tach, so probably temporarily hooking up a shop tach would be best. The calculation works for any tire size you have. If you've changed the rear end you would need to know the ratio. If you do this properly, you can make note of how far the ribbon is out at a few different speeds, maybe even add a couple of inconspicuous dots on the panel for common city and highway speeds.

Doing some reading, speedometer calibration is dependent on a gear with the right number of teeth in the speedometer cable drive box at the rear. There were apparently three or four common sizes of coloured gears used with various combinations of diff ratios and having an OD, as noted in Phil's writeup. Anything other than the stock tire size will of course be a significant factor in how far the speedometer is out. Manufacturers like Volvo often factored in a few mph as a safety margin, knowing that most people have a tendency to go a bit over.

I recall that there were once speedometer shops in my area who could clean and do some kind of adjustment of those speedometers if the error started to increase with age or the cylinder (ribbon) was misbehaving. It wasn't trivial or cheap, so I just learned to live with it and mentally remember to add a few mph based on a few tests using multiple chase cars of friends. I vaguely recall they had to crack the panel open to get at the spring and bushings, maybe even replace the little magnet inside, beyond the realm of DIY repair.

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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now