Short answer
ASTM D975 is the Standard Specification for Diesel Fuel used in the United States. It defines seven grades: No. 1-D and No. 2-D, each in S15, S500 and S5000 sulfur levels, plus No. 4-D. The number describes the type of distillate and the S-figure is the maximum sulfur in parts per million.
The grades
No. 1-D is a lighter middle distillate with higher volatility than No. 2-D. It flows better in cold weather and is often blended into winter diesel. No. 2-D is the general-purpose grade for on-road trucks, equipment and generators, and is suited to engines that see varying speeds and loads.
No. 4-D is a heavier distillate, or a blend of distillate and residual oil, for low- and medium-speed engines running mostly at constant speed and load. It is not a fuel for modern on-road engines.
S15, S500 and S5000
The S designation is the sulfur limit: 15, 500 or 5,000 ppm. S15 is ultra-low sulfur diesel (ULSD), which US rules now require for highway, nonroad, locomotive and marine diesel engines. S500 corresponds to the older low-sulfur grade, and S5000 to the older regular-sulfur grade.
What D975 controls
Requirements include flash point, water and sediment, distillation temperature, kinematic viscosity, ash, sulfur, copper strip corrosion, cetane number (at least 40 for the No. 1-D and No. 2-D grades), aromaticity, carbon residue and lubricity. The lubricity limit for No. 2-D is a maximum wear scar of 520 micrometers in the high-frequency reciprocating rig (HFRR) test at 60 °C.
D975 does not set one national cloud point, because climates vary so much. Low-temperature operability is left to seasonal blending and agreement between buyer and seller, with guidance in the standard.
Biodiesel within D975
D975 allows up to 5 percent biodiesel (B5) when the biodiesel meets ASTM D6751 and the finished blend meets D975. Blends from 6 to 20 percent fall under a separate specification, ASTM D7467. Because B5 and below is still D975 diesel, it may not be labeled as a biodiesel blend at the pump.
Frequently asked questions
Is “red diesel” a different ASTM grade?
No. Red dye marks fuel as untaxed for off-road use under tax rules; the fuel itself is still a D975 grade.
Does D975 specify how long diesel can be stored?
No. Storage life depends on the fuel, temperature, water and microbial contamination. Stored fuel for generators and equipment is commonly tested before relying on it.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
