Quality of diesel fuel in any given area of refined product distribution is mainly driven by crude feedstock and refinery operations. Low cetane diesel is comprised of more aromatics whereas higher cetane diesel has a higher parafinicity index. And now in laymans terms, high aromatics are due to limited processing of certain crudes (all crudes are not the same). Early diesel production was a straight cut off the crude stills with very little additional processing. Except for third world countries, this is pretty much in the rear view mirror due to our environmental regulations. Low sulfur requirements require additional processing to remove sulfur; the predominant process is hydrotreating. This does a good job of sulfur removal and also yields a higher quality diesel due to saturation of the hydrocarbon molecules with hydrogen. This converts some aromatics to paraffin compounds. Paraffins are higher cetane components. The fuel economy game is not solely driven by cetane (though it helps by assisting ignition). The amount of energy in each diesel gallon can vary from refinery to refinery based on their processes and blending of other components into the diesel "pool". Diesel fuel has components in the 430 to 650F boiling range. As the percent of "heavies" (550 to 650F) increases, so will you fuel's energy content. Conversly, if the fuel has a high percentage of "lights" (under 500F), the energy content will be less and result in reduced MPG. Moral to this story is to find the best fuel in your area and stick with it. In my location our diesel fuel comes from either Phillips 66 or Valero. Being retired from P66, I am very familiar with the processes in that plant and they yield an excellent diesel with an average (yes it can vary a bit) cetane rating of 46. Valero also produces good diesel, every bit the equivalent of the P66. Amarillo, TX is referred to as the World's Largest Truckstop no doubt due to the quality of fuel available. Directly east into Oklahoma the diesel is lousy and out west in New Mexico it's not much better.