Although filling an entire trunk full of peanut butter, let’s say 500kg worth (assume a 450 litre trunk and density of 1.1 kg/l according to Google), then adding all that weight over the rear axle would affect the handling and balance of the car, potentially making it dangerous and therefore illegal to drive.
So, it’s not actually that clear cut that it wouldn’t be illegal!
My mom's from way out in the country. The first and only time she was stopped on suspicion of drunk driving was when she had an entire butchered pig from her parents in the back of her station car making it swerve.
And that was less than 1/4 the weight of that peanut butter collection 😄
Most SUVs cannot handle the weight, let alone a car. A typical SUV will allow for 200-300 lbs in the trunk. At least according to the GVWR and with 2 passengers.
Peanut butter does work very well for the classic high school physics egg drop competition. It's definitely not standard safety equipment but could work in a Jif.
Whoever is down voting me should actually go out and check what the GVWR of their vehicle is and the curb weight + fuel and passengers. It's surprisingly low even for a full sized SUV unless you own a body on frame SUV. Sure you can exceed the GVWR but you are potentially endangering others on the road with increased braking distances and poor handling.
You should look up those values on SUVs vs. regular older cars. I can almost guarantee that many older cars allowed you to transport more than most modern SUVs.