I believe I found an easy way to significantly improve prius soundproofing, which was a main concern for me at speeds over 70 mph - with stock tires the road noise was too much to tolerate, you actually have to raise your voice so your passengers can hear you. I was also trying to reduce the typical Atkinson engine droning noise from the rear compatment that can be heard at patial throttle I purchased a set of Weather Tech floor mats and 3 sheets of dynapad soundproofing material from Dynamat. I placed one sheet of dynapad over the spare tire inside the rear compartment, and two remaining sheets were cut to exact fit and placed on the floor under the weathertech floor mats. I was very much pleasantly suprised with the results - the tire noise and engine drone were reduced significantly, we could actually talk at speeds of 80mph. The total cost of this mod was about $500.00 weathetech mats included in this price, and I did not have to take anything apart.