Well, I did the test with the bike's speedometer which has an RPM function in it & it all went out OK.

It wasn't very accurate but gave a good rough figure because I was holding the sensor in my hand trying to hold the required distance from the magnet (3mm +-1mm).

The test results:

the big Motronics has a speed of around 1800 RPM (dancing between 1772 & 1800)

the moderate EG&G which looked/sounded very fast, has a speed of around 3600 RPM (the speedometer is rated 3600 RPM max & I saw speeds of 3720 RPM)

All the tests were made with the motors standing vertically on the table.

Thanks for all the help !
cnc2.