Virtually all GPS units display speed, and even in the 6 volt trucks a tiny 12 volt motorcycle battery will last for weeks between recharges if it only powers the GPS, cell phone charger, etc. A solar trickle charger will keep it powered up for practically forever. My Garmin lets me select between MPH and KPH with a simple programming change.
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!