How long does a car last when idling with a full tank of gas?

A rule of thumb we used was: a passenger car burns about 0.5 gallons of fuel per hour while idling. If you have a 12 gallon tank you can figure it will idle for 24 hours.

The range is large depending upon the engine, loads and calibrations. if you have air conditioning running your flow rate will be double so your tank will be empty in 12 hours.

It depends on the car, the engine size, carburation, gas tank capacity, and any accessories that might be running.