J Burns wrote, on Wed, 10 Sep 2014 21:36:21 -0400:

I suppose they mean not to keep it in your trunk. When I fetch gas in
the car, I like to keep it up front, where I can see or smell any problem.

Legally, in California, you can carry up to 600 pounds (yes, pounds) of
hazardous materials in your car (or truck) without needing a special
permit. That's more gasoline than you can carry.

In NY, I believe it's 25 gallons, which is less gasoline than you can carry.
I can carry about 50 gallons in my trunk, for example.