Prius Gas Millage Dropped?

When I bought my Toyota Prius I was averaging about 52 to 54 miles to the gallon. It stayed that way till I hit about 35,000 miles now it has dropped to about 45. The only thing I did was change the tires, but I maintain the air pressure. I have several friends with the Prius and they are still maintaining high 57 to 52.They all have ethanol in gas where they live too. Does something happen to the engine as miles are applied. I have gone to the dealer ship so many times they have no idea. I would love any impute, even though 45 is not bad I want it back to the mid 50’s. Thanks