What it measures. The diesel (ULSD) crack spread approximates a refiner's gross margin for turning a barrel of crude into diesel/heating oil: the NYMEX ULSD futures price, converted from $/gallon to $/barrel (× 42), minus the NYMEX WTI crude futures price. Daily settles of the continuous front-month contracts. The left axis shows % change from the first day in view. Stats reflect the visible window — drag the chart (or the time axis to zoom) to re-scope them.