I took my 03 Prius to the dealer for a 15,000 mile maintenance and to replace 1 blown and 1 about-to-blow tires. Before the maintenance, I was getting 45-48 mpg (51 mpg on the freeway). Since then (350 miles), I have not gotten more than 39 mpg. A mechanic at the dealership told me it was because of the new tires, which were the recommended Bridgestone Potenza. The BS meter was way off the scale. The dealership is not helpful. Any ideas?