Looks like failure is going to have to send his script to you.
There probably are a lot of "speedometer" scripts out there. While you're waiting for failure to send his to you, you might try something I wrote a couple of years ago for a couple of the artists at my place of work, which you can get at this link. It does the job more cleanly (fewer connections and calculations), and is more "idiot-proof" (i.e. error checking).
Download it and put it into your scripts directory, then go into Maya and load your scene. To facilitate use of the script, I'd make a shelf button with the command, "source mcSpeedAttribute; for ($sSel in ls -selection) mcSpeedAttribute($sSel, 5280, 24);" (don't include the quotation marks, obviously).
Then, select all objects to which you wish to apply a "speed" attribute, and press the shelf button (or execute the command above if you don't make a button).
Incidentally, the 5280 and 24 in the command above refer to units (feet per mile, in your case) and frames per second, respectively. Alter as required.
To remove the speed attribute from an object, use a value of 0 for either the units or frames per second.