Join Date: Mar 2006
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Frankly, there is very little difference in the heating value of ULSD (15 ppm sulphur) and LSD (500 ppm sulphur). The difference is so small that you should not be able to tell in your mileage.
Your power should not decline with ULSD. Since the increased refining actually works to raise the cetane of the diesel, you may notice reduced engine noise and faster starting.
You may have some problem that is decreasing your mileage and performance, but the problem is not related to the LSD to ULSD switch.