2000 jimmy voltage gauge fluctuation

i just bought a 2000 jimmy 4x4 with about 89,000 miles. when i turn on my blinker i notice my volt gauge fluctuate. i've also seen it go down by itself to about 12v then back up to 14 by itself with the engine idling. i've already had my battery and alternator checked because of this and they checked out o.k. Is this common??

Also when i rev the engine in park my speedometer needle goes up to about 10-15 mph when i reach 2500 rpm. my odometer starts counting as well as if i'm really driving. Is this something to worry about?