Over the last month I've noticed a 10 mpg decline. I was getting around 54mpg now I'm getting 43mpg. I have an 05 Prius and I just hit 30,000 when it started to go down.

I live in Wisconsin and it has been getting colder but it seems like a large decline.

Has anyone else noticed this? Is this common during winter months or is this something I should have checked out?

Thanks