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This shouldn't be too difficult to figure out if you have a gauge that reads mpg.
When you are coasting in neutral read the mpg figure and note the speed at the same time. Because you now know 1) miles per hour and 2) miles per gallon and 3) your engine is at idle, it should be easy to calculate gallons per hour at idle.
It's late now but I'll work out a formula tomorrow.
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