You have to ask yourself, what is your context? If you write about big cars exclusively, then your site's title should be "Big Cars". When a person lands on your home page, the most important thing about that page is that it is about Big Cars, so for your home page the <h1> will have Big Cars. Your home page might have multiple posts on the landing screen, but the individual posts are not important on your home page. Hence each individual post title on your home page gets an <h2>. However, once you go to an individual post, say "Engine Trouble", you want that post to be found when people search for engine trouble. Of course, you also want it to be found for big cars, but that is not the most important thing on that page. So, for individual posts / pages, the post / page title is put in an <h1>, while the site title is pushed to an <h2>.