Why does my car sound louder than normal on the freeway?

I have a 2007 toyota yaris hatchback, I have had the car for 4 years now with NO problems. I got my oil changed about six weeks ago and shortly after someone rear-ended going about 15 MGH. There is no damage to the vehicle that i can see, however i am no mechanic. I have however noticed a decrease in gas mileage by about 20 miles per tank, (I went from 300 miles per tank to about 280) on and when i am on the freeway it sounds like i am running into a lower gear although i not. I am wondering if there is anything i can try to narrow down the possible cause as the dealer wants $ 100 to even look at it (thank you extended warranty…) Or if anyone has had this problem and knows whats wrong? Thanks!