My car used to get about 500-525 miles a galloon, but only gets about 400 now?

What is your question?

This was after a car accident, but it seems to be getting less milage now. I know, because I always pay attention to the miles my car has left, and see how many miles I've driven since that last time I've filled up. I also haven't changed my driving style up, it's been the same commute as always. Any tips on where to get started first? I want to avoid taking it to a dealership, since I'm short on cash right now.