bad gas mileage

FWIW: I have been getting better mpg on my 2008 Touring Pkg5 since getting the Michelin LRR tires and running 42-F/40-R psi as the tires were used a few k miles. The type of gas and brand doesn't seem to matter as much as being sure the right tires are installed and tire inflation is maintained (above Mfr minimums) and of course maintaining as steady a speed as possible with few start/stops and little or no hard acceleration. We plan to get another Prius after this one is ready for trade-in, but that may not be for awhile as I only just passed 80k miles.

Good luck. Don't give up. Get the dealer to put better tires on your Prius or do it yourself. And if you still want to trade it in, check out the newer Prius model years -- they post even higher Fed MPG ratings.