Motor RPM Checker
Here is my small device for checking the RPM of Meccano motors. The photos show an E20R attached to the device, which is based on the Standard Mechanisms S. M.40 (USA edition). It provides a basic 243:1 ratio on just two shafts, which I have increased to 5103 by adding extra gearing. The extra gearing is on one side of the unit, with a crank on the other side to indicate the approximate number of revs. A dial could be added if accuracy is required.
PS: I started off believing that the S. M.40 could not work, but by adding washers in various positions on both shafts, discovered that it does!