I'll apologize in advance for a dumb girl question...I have owned other cars in the past, but none as new or fancy as my Prius.

Two weeks ago, I bought a used 2004 prius with 27,500 miles on it. I thought the tires were a little low so I went to the gas station to check them tonight. The tires were each at 30-31.5 psi, and the max listed on the tires is 44, so I bumped them all up to 40-41.5 psi (it gets so hot during the day, I don't want to risk getting them any closer to the max). I am hoping this will help my FE, which has been pretty dismal so far, even for a newbie.

While I was adding air, I noticed that all four tires matched and said "Integrity" (I assume this is the brand name?) Can someone who bought their 2004 new tell me if these are the original tires to the car? It seems like they have very little tread left on the most raised area (I'm almost sure they will fail the penny test). Does this mean I need new tires already? How long do the original tires typically last? I have heard of really cheap 40,000 mile tires, but tires that are worn out by 28,000??? How long did most people's original tires last? The car was originally purchased in March of 2004. Thanks in advance!

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Hillary