The difference between the 6 and 8 cylinder cars in terms of fuel mileage is small.... I would go for the 500 over the 300, as its got more power, and will return almost the same fuel mileage.

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-Justin

91 560 SEC AMG - other dogs dd
01 Honda S2000 - dogs dd
07 MB ML320 CDI - dd
16 Lexus IS250 - wifes dd

it's automatic.