**Clarification**

When the car is stopped and the engine is running, that condition is generally referred to as "idle".

To perform the idle test, drive until the engine is warm, stop the car, but leave the engine running.

Record the distance driven and check the mileage ("double-click" the segment button as you start a stop watch). This is the base mpg.

After a period of time (say, 5 minutes), double-click the segment button again and stop the stopwatch. Record both the time from the stopwatch and the mpg (the mpg doesn't seem to update at one-minute intervals while at idle).

From the base numbers, you can calculate the total number of gallons used before the start of the test: (e.g. 18.7 miles/53.8 mpg = 0.34758 gal).

Calculate the number of gallons used at the end of the test the same way: (e.g. 18.7 miles/52.6 mpg = 0.3555 gal).

Subtract the 1st number from the second number to get the quantity of gas used while idling (0.3555 - 0.34758 = .0079 gal).

Divide the number of gallons used at idle by the time from the stop watch to get the gallons per minute, then multiply by 60 (minutes/hour) to get the gallons per hour used at idle (.0079/5 *60 = .095 gal/hour).

The longer the test, the more accurate the results.

If you repeat the test with the AC (or maybe your 1000 watt amp) on, you can see how that affects gas mileage.