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There is at least one specific set of geometric parameters for a spur gear designed in the imperial system. This set is N = 7, DP = 6, CP = 0.5236 ”, α = 27.6 °. Such a gear, converted to the metric system, has a non-standard module - m = 4.2333 mm. It turns out that when it is not known in which system the recreated gear was originally designed (metric or imperial), it is possible to determine from the measurements of this gear the standard parameters of the gear identical to the nominal one for the metric system - e.g. module m = 4. The authors show that there is a risk of inability to recreate the correct parameters of the nominal gear (even with the use of alternative parameters) and the possible consequences of this. The recreated values ​​are m = 4, y = 1, x = +0.2525, α = 20 °, i.e. a standard gear with non-shifted teeth and a standard pressure angle. Meanwhile, the alternative parameters identical to the nominal ones are, however, m = 4, y = 1.0215, x = +0.2410, α = 20.2985 °.