I wasn't meaning the avg mph a Garmin uses when calculating distance to destination, but the Speed Limit it posts on-screen when going through posted areas. My Garmin shows both speed I'm traveling as well as what the posted speed limit is wherever I am.
I'm always impressed that it changes within feet, it seems (probably a tenth of a mile on the highway), whenever the speed limit signs change, whether on the Interstate or going through town. Has to be connected to GPS coordinates that are submitted by DOT, that are then part of the updates we do as users, then relayed by our own positions via GPS satellites.
Just another one of the mind-wandering questions that I ponder when driving.