I can only think of two ways to know what your fuel consumption is during idle, well maybe three.

1. Fill up your fuel tank. Make note of the time, start the engine and idle until the engine runs out of fuel. Now you can calculate how much fuel you use per second, minute, hour, etc...

2. Bypass your fuel cell and connect a measured gallon, liter, etc... and measure time it takes for engine to run out of fuel. Less accurate than #1, but much cheaper (unless you set the Jeep on fire).

3. It's possible that Chrysler has published a fuel consumption information.