I was adding motion blurred cars to a project recently and since I already had the camera shutter set, I figured it would be easiest to start with a car going a reasonable speed rather than just trial-and-error to get the blur right.
So here is a quick Maxscript to show you the speed of an object. Optional units are miles-per-hour, kilometers-per-hour, feet-per-second, meters-per-second and knots. It works with any system units you have set except “generic”. More