I have a 2004 Sienna XLE Limited, and I typically get 16.4, may of 18.4 on long highway trips, never hit twenty. I have not raised the air pressure, but original Bridgestone tires only lased 21k. Recently replaced with Yokohama Avid TRZ's. What are the ramifications of raising the pressure? (By the way I typically get 33.5 on my TCH, same local style of driving)