I experienced some muffle sound in both front and rear seats. even the speed is around 25mph. Mine is Prius V with 17 inch wheels all-season Michelin tires. though the sound is quite small and low, I would really want to get rid of it.
Road noise if a big function of the surface you're driving on, so the issue is how well does the 2010 Pri supress noise on rough pavement, or what is the range of dB sound level on many different roads. I know it's fairly quiet on smooth pavement.
