Factual error: Season 5 and they are still driving cars around! Gasoline won't last years unless specially treated.
Other mistake: Most if not every time someone kills a walker in close proximity or repeatedly bludgeons one that is already dead with a blunt object and there is a CGI blood spatter effect (as opposed to a practical blood spatter effect), the blood magically never gets on the person's face.
Answer: Walkers are attracted to noise, they can tell who's dead and who isn't dead, in S1 when Rick and Glenn cover themselves in walkers' guts they're able to get far by walking through the herd of walkers because they smell like they're dead, but as soon as the guts get washed off by the rain the walkers immediately attack them, if you draw attention to yourself they will attack you which we've seen that many times throughout the series that it's not always successful. Whether you're covered in guts hiding under dead corpses or under a car.