I'm not a huge fan of going either-or for gas versus spare tire. Both are useful. Arguably, in most instance, the spare comes in handy more than the gas does.
That said, if you insist on carrying gas in lieu of a spare tire, I'd rather see something a bit more secure instead of relying on a strap, especially if you're going to be running this thing on the road.
From Expedition Exchange:
From Adventure Trailers: