Let's add some reality to your fiction -
gas goes bad. A couple months after you buy it the gas will start to turn into gum and varnish, which will cause major problems by clogging the fuel lines and filters. Yes there are fuel stabilizers you can put into the gas to extend the shelf live up to 15 months, I add it to the two 5 gallon tanks I fill up at the start of hurricane season, however the gas stations would not have put that in before the outbreak. According to
The Walking Dead timeline when Rick wakes up at the start of the series its already been 2 months since the outbreak, so their vehicles would soon start failing due to bad gas.