Another option for speedometer checking would be a GPS unit. If the truck runs a 6 volt electrical system there are inexpensive voltage doublers available that can handle the minuscule current draw of a GPS.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!