Originally Posted by Tightwad
The SMV sign is a requirement for all vehicles that can not
travel at minumim posted road speed in most states.
That's not quite true. It's primarily meant for farm implements on roads, since farm implements don't meet car's standards for safety/visibility equipment (brake lights, turn signals, etc.) Here's the Michigan vehicle code:
MCLA 257.688(g)
Slow Moving Vehicle (SMV) emblems and additional reflectors are required on any implement of husbandry, farm tractor, special mobile equipment, and every vehicle with a maximum speed potential of 25 miles an hour operated on public highways.
I'm not a farm implement, or "special mobile equipment" - i.e. backhoe, bobcat, etc - and I often exceed 25 mph, so I don't meet the definition. So use them if you wish. There's no prohibition on using them if you don't meet the criteria, but it's not required.