Tom's point also deserves real consideration. You never get to burn properly the fuel being fed into the engine under your circumstance. At the bottom of the hill the engine could just be cleaning itself out as it returns to work. I think the smoke should favour black in that situation yet overfuelling produces white smoke sometimes I am led to believe.

If the car operates well in other circumstances as Tom also mentions I would ignore it at present yet keep an eye on it over time. It could well be if we submitted our own diesels to that hill results may be simular.

You could go buy an ebay special and do a comparison drive down the hill and let us know. Most the offerings on ebay are already going downhill. Now at last I understand why many of them get at least 40mpg. If they mentioned the cars emmisions at the bottom of the hills this might be put to rest. Unfortunatly they always leave that part out.