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There is a tendency to misinterpret cetane level as an indication of the fuel's energy content. But a level that is higher than required will not enhance engine efficiency.

"Just because a fuel has a high cetane number does not mean it will provide optimum performance," says Harvey. "A high cetane number can - but not necessarily does - suggest a lower BTU/gal. if the No. 2 diesel fuel has been blended with kerosene to achieve the higher number."

Gault adds, "Generally the higher cetane number fuels are less dense, and therefore have lower fuel economy, so there are tradeoffs. EMA recommends a minimum cetane number of 47 for the best combination of performance and fuel economy."

Cold weather performance
Cloud point and pour point directly affect cold weather performance. "During the summer months, cloud and pour points do not carry the importance they will for winter months," says Harvey. "For winter operability, fuels exhibiting low cloud and pour points can generally provide a wider range of operability temperatures, especially at low ambient temperatures."

The cloud point is the temperature at which crystals of paraffin wax first appear. (This creates a cloudiness in the fuel.) These crystals collect and plug the fuel filter. The pour point refers to the temperature at which the diesel fuel will no longer pour; it becomes too thick.