13 KWHr per US Gallon, depending on the specific gravity.
A little less with a turbo.
When comparing specs from different manufacturers, check the SG of the test fuel.
When comparing specs. I found one brand of engine apparently used 10% less fuel than all of the competition.
On closer checking I found that they were testing with heavy fuel. 10% heavier!
This is based on a loaded set.
The efficiency drops off as the load drops so Hoxton's number is also valid.
When fuel consumption is an important selection criteria, the consumption at 1/4 load or whatever light load is listed may be a more accurate indication of overall running cost.
I often find sets running at 10% to 25% of rated load, however the extra capacity is needed for occasional peak loads or for starting large motors.
Fuel consumption will depend more on normal or average loading than on peak loading.
Bill
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"Why not the best?"
Jimmy Carter