Since the US switched to ULSD, the refineries have been able to export the fuel to Europe and other ports where the ULSD was already standard. Before ULSD, we had an excess supply of diesel that kept prices low. There is still some seasonal differences with the price inregards to heating oil use, but the price difference now is mostly because of that ability to export the fuel.
The difference amount is regional. It's around 60 cents here. In California, diesel and premium gas are about the same. Sometimes it drops to regular prices there.
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