I have a 2000 ML 430. It's hard to compare different model years. Mine has a 3.46 rear gear, and I believe yours has a 3.70. Yours is also heavier. However I drove 5000 miles on a trip this summer in the western US across deserts and up and down mountains and averaged 19+. Headwinds seem to kill the mileage on these, however tailwinds seem to really help. I went from 17.3 to 19.6 in just 150 miles with a good tailwind in Montana. It seems to like higher altitudes too as the fuel management system seems to cut the power down! I see you're from Florida at sea level (or below) and no hills, so it's hard to compare, but I would take it to the dealer in any case, because it just doesn't sound right to me!!

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David L