I think the recent price fluctuations have been because of taxes and supply. The price of diesel has always changed with the seasons though because of the heating oil issue that TXBill brought up. The oil refining process separates crude oil (or sometimes gasoline condensate which is almost as dense as crude oil) into levels of weight (ie. gasoline, lubricating oils, kerosene, diesel, etc., not in that order) and the weight and density of diesel is very very similar to home heating oil so in the winter, there is a bigger demand and thus the higher price. In the summer, there is a large surplus because of the lesser demand for those oils and so the lower relative prices.

This past year or so was strange though in that the fuel prices didn't seem to change in the same proportion that they always have.

Alex

__________________
1983 300D (parked for four years)
2012 VW Sportwagen TDI Manual
2001 Miata SE
1962 Chevrolet Corvair Rampside