Any can rated to comply with the CARB (California Air Resource Board) is rated to completely contain gasoline fumes when the gasoline is not being poured. However, some gasoline canisters are also designed to "vent" excess pressure caused by temperature changes. If you've ever gotten into a car that has been parked in the full sun on a hot day for a while, then you can imagine that there's a possibility that some lower quality jerry cans will vent. If you're still interested in getting a jerry can, about the best ones I've seen are Justrite Mfg.'s UNO® D.O.T. Safety Cans.

I've also heard of a couple products that are better suited for emergency spare gasoline. One of them was called "Spare Tank" and the other is "dry gas" (which is actually just ethanol or methanol, I believe, and is mainly used for removing suspended water from gasoline). You may want to search the 'net a bit on these alternatives, as it is still tough for me to recommend carrying spare gasoline in an automobile… especially in the passenger cabin like in an SUV.