If you and your wife tend to keep at least half a tank of gas in each of your vehicles, and you have a siphon, you would probably have a full tank for one vehicle.
I do have to disagree with that part. Personally, I would never ever count on being able to siphon fuel out of a vehicle’s fuel tank unless I know it can be done with that particular vehicle because of doing it before. It seems that more vehicles have anti-siphon systems than not. The easiest method for these vehicles is to use the vehicle’s own fuel pump and take the fuel out of the fuel injector test port.
Everything else Susan said, though, is spot on (as usual).