I know in Hollywood movies and tv shows gas tanks blow up as easily as shatterng a light bulb but in real life it takes a lot of effort to make them blow. You see liquid gasoline does not explode you need to have a fuel VAPOR/Air mix and you need to be rather precise in the ratio. Oh you can get it to burn even if the ratio is off.. but to explode.. Well if you are old enough to have played with a carborator's adjustments you know if you get it too rich or lean it runs poorly and that's just a tiny bit off.
Mythbusters tried and tried to get a Hollywood like gas tank explosion and finally did it the way Hollywood did it.. Attached explosives and a remote detonator.
But if the tank leaks and it runs out on the garage floor.. Well gasoline will vaporise at common temps (even at darn cold temps) and then all you need is a spark.. Say a switch closign or opening or a furnace in the garage kicking in (or water heater) and KABOOM.