Dash/speed in park

when my car is in park and i rev it the mph on the dash spits out speeds. have gotten it up to 15 mph while in park. what could be the reason for this? i know my speed is off by 5 mph, could these two problems be connected in any way?

my 94 caprice classic does this too. best i got out of it was 4Mph... (mines the digital gauge)

http://blackevilweredragon.spymac.com/mycar/5.jpg

ya mine is digital also...anyone know what this could be?

yea, im surprised no one has an answer to this one.. usually everyone has an answer to everything. :P