I have been researching the legality of carrying a spare fuel can in my car since someone said it was illegal. Where I live, and probably everywhere, it is legal to carry less than 30L of gasoline in a jerry can in your car. I have been safely within regulations with 10L. At any rate, I had never run into any legal issues with carrying it in my car. My problem was with gasoline vapors escaping the can in warm weather. Other people have remarked about a faint odor of gasoline in my car, something to which I had become a bit "nose-blind" or accustomed to the odor and not noticing it. I tried deodorizers and heavier duty fuel cans, but the problem persisted. I believe the gasoline eventually migrates through the plastic. I have mitigated the problem somewhat by using a heavy metal can with a tight seal. I am still detecting a very faint odor of gasoline at times, but nothing like before, and certainly not a problem at all. I have placed the can in a cardboard box, also containing charcoal pads, which have reduced the odor considerably.
i used to get an odor just filling the lawn mower can and driving home one mile. definitely don't like that in my car.
NATO type Jerry cans are roughly 20L ( 5 US galons) Our recent 4000+ miles through >½ dozen states in the 5th wheel, we had about 8 gallons gas for the generator & 5 of diesel fuel for the Duramax. But it was cable locked into the bed of the truck. Better to get one of those trailer hitch cargo carriers for the back of a car then to keep it inside the passenger compartment. Especially if they're plastic jugs because the fuel fumes do eventually weep right through the walls. Actual steel Jerry cans, not so much. But those suckers cost a fortune nowadays.
My 10L metal fuel can was not cheap, it cost me $53, (and was from Aliexpress no less). It was the second one I got which leaked. I returned the first one, but I decided to repair the second one myself. I knew about a product called "fuel tank sealer" or something similar, but it was quite expensive and only available in a large container in my area. I used a mixture of acetone and Seal-All, which I spread around inside the can, and left it outside to evaporate. It seems to have worked, but perhaps I will give it a second coat some day.
Yeah I should have said European or USA made jerry cans are expensive, as the Chinese stuff are complete junk. The USA spec ones are often coated inside whereas the Chinese version will Rust. That's from personal experience. The above link is a comparison from the Chinese junk versus the good stuff.
Mine performs well enough for "cheap Chinese junk." Not everything made in China is substandard. It's just that they sometimes have to build down to a price for their clients. Mine was a good effort at that.
I haven't had to do this in years since I went electric on all our lawn care tools but when I did I would simply place the 5 gallon fuel can inside a large lawn plastic bag after filling and cinch it up as tightly as I could. I can also say spilled gas inside a car will make for a very hard and long term problem it if soaks into the carpet and insulation (which is almost immediately). BTDT One can purchase a rotopax style exterior solution (with locks) for the Prius from Prius off-road.
Some Jeeps carry them on the back end of their 4x4. Some similarly shaped SUVs carry them on the front. Pick your poison .