Our 2002 Prius has been all over lately. We had the traction battery pack replaced in July 2013 with one from ReInvolt in North Carolina. In October they were bought out by Dorman Electronics. In June the traction battery pack threw several errors and had a SOC reading indicating a failed pack. Dorman replaced it under warranty, although we're STILL fighting a month later with paperwork to get the installer reimbursed for the installation labor. I'll call Monday to see if they finally got paid.
Anyway, before the battery went bad we were seeing a solid 43mpg city and 45mpg highway. After the pack was replaced I had to replace the MAF as it was throwing error codes and causing the computer to shut down the engine. On two highway trips this week of 250 miles each we saw 47 to 48mpg on the highway. I'm concerned though as mixed driving today on a fresh tank of gas yielded only 40mpg. I may replace the O2 sensor next.
We're on the original transmission and hybrid drive system and pushing 125,000 miles on the car.