Probably with Math But don't ask me how, I can barely spel

That and I don't know if you are looking for RPM of the object or a linear speed of a point on the circumference of the object... In fact, I am unsure if there is a difference

Thankfully there is a Google for that!

Or continue to wait for a math geek to reply... but at least I bumped your topic

You can use the node context in a function node to save the previous sample and time in the context. Then you can get the time difference in milliseconds by subtracting the times and the distance move by subtracting the positions. Then the speed is distance/time. See the node red docs page on Writing Functions for more information on using the context.