I have a 2005 Prius and used it for long trips and I drive on hot days, being in Texas. I have lived in CA and driven I-5, if you drive a Prius like the most of traffic, you will not see the high MPGs.
However I can give you some tips:
1) Set your A/C temp to within 20 degrees of the outside temp, this lets the system step down its speed and load until it cools the car down. ( The A/C compressor is run by a motor using battery power). 2) Drive in the right lane on the freeway and set the speed control at 70. On long trips when my wife would drive, she would keep up with traffic, drive 75, change lanes to pass trucks and the MPG would drop to 39 to 43 MPG. We would switch seats and I would drive and stay in the right lane and drive at 70, when we pulled in to fuel up the MPG Avg would 49 to 51 MPG. I have seen 59 MPG after 90 miles on a two lane road after driving from Kitty Hawk, NC down the coast. 3) Never and I mean never change the size or profile of the tires. A tire store near me told me of an Prius owner who needed new tires and had 205 profile tires installed (195 is profile from Toyota), he came back after two weeks and had them install 195 profile, it seemed his MPG was cut in half. 4) Double check your tire pressure, my Prius calls for 35 psi front and 33psi rear, 7 to 10 lbs low can play heck with your MPG. I keep mine at 35 psi front and rear and I had many a time when a oil change tech would lower it to 32 psi on a hot tire because that's what all the other cars are set at and when it cooled and the pressure would 21 or 22 psi.