If you do try this cockamamie scheme (which I certainly don't recommend), then for the love of God, only transport FULL gas containers in the back seat or trunk.

The most explosive and therefore most hazardous part of transporting gasoline is the vapor. Don't create a lot of vapor in the containers - fill them up full.

You know what? Don't even do that. Pay a few extra euros for gas the right way and try to find your savings somewhere else. Sheesh. /rant