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I would assume .33 to 1.0/gls hour for most any motor, up to and including the 15L Cummins in my work Peterbilt.
But, what would be the point of idle rpm fuel consumption measurements? I'd think "engine wear without work performed" would be more useful way of thinking about it (and there are scenarios where providing heating or cooled air for passengers is if high value). Engines are designed with a number of hours of economic life as a gauge. Right down to a two-stroke weedeater (cheap one at 250/hrs) to the light duty diesel in my pickup (10,000-hrs) not just commercial calculations.
The percent idle time is a recognized metric. A mph averageotherwise. On my pickup brand engine if one keeps that average at 27-mph or higher then FE is relatively good. The 10K engine life expectation (mean time between overhaul) is predicated on a 35-mph average.
The "usefulness" of engine idle fuel consumption [EIFC] ought to be related, somehow, to performance (achieving work). In a hurricane evactuation, for example, I'd expect that -- small to large -- personal vehicles are going to do no better than 3-5/mpg in bumper-to-bumper traffic. EIFC is not important in this, it is fuel capacity that matters far more (and, as note, the ability to travel 150-miles inland to be able to re-fuel from undisturbed stocks).
On the work Peterbilt we engage "high idle" if idling is to be extended past five minutes per the manufacturer instructions. Engine longevity/reliability far exceeds reduced consumption at this low level.
So, what am I missing as to importance, here?
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