Just came across this about the supply situation : -
'Kampfgruppe Peiper carried enough ammunition to last it four or five days and fuel for an even shorter period, as two entire trainloads of gasoline never made it to the assembly points. Additional supplies of fuel would have to be captured, taken from Allied dumps'.
This from Astor's
'A Blood-Dimmed Tide' ( OK, not the greatest source but it happens to be the one I'm reading at the moment
)