What the ASTM numbers actually mean
ASTM International publishes the fuel specifications that engine manufacturers reference in their warranties. There are three relevant ones:
- ASTM D975 covers conventional petroleum diesel. Since 2008, it has permitted up to 5% biodiesel in fuel still sold as plain diesel. That's why a station can sell B5 with no separate label.
- ASTM D7467 covers blends from B6 through B20. If you see a pump labeled "B20" or "Contains up to 20% biodiesel," this is the spec it should meet.
- ASTM D6751 covers B100 — the pure biodiesel blendstock. Refineries and blenders use D6751-grade B100 to make D7467-grade B20. End users running B100 directly need to confirm both the fuel and the engine.
If a fuel meets the right ASTM spec, OEMs treat it as eligible under their warranty terms. If it doesn't — for example, a homebrew B100 that hasn't been tested for cetane, water, or oxidation stability — engine damage from that fuel is generally not covered.
Energy content and fuel economy
Biodiesel contains less energy per gallon than petroleum diesel because the molecules carry oxygen, which doesn't burn. The AFDC fuel-properties chart lists petroleum diesel at about 128,488 BTU per gallon and B100 at about 119,550 BTU per gallon — roughly 7% lower.
In practice:
- B5: no detectable fuel-economy change.
- B20: about 1-2% lower energy per gallon. Most drivers don't notice it; on a 500-mile tank, that's about 5-10 fewer miles.
- B100: real-world fuel economy drops about 7%. Drivers running B100 do notice it.
Lifecycle emissions: where the 86% number comes from
The widely-cited "biodiesel reduces emissions up to 86%" figure comes from Argonne National Laboratory's GREET model. The full range, accounting for feedstock and land use, is more nuanced:
- Biodiesel from waste cooking oil and animal fats: roughly 70-86% lower lifecycle GHG than petroleum diesel.
- Soybean, canola, and carinata biodiesel: 40-69% lower, after counting land-use change.
The takeaway: biodiesel is meaningfully lower-carbon than petroleum diesel across the board, but the spread between feedstocks is wide. If lifecycle carbon matters to you (fleet sustainability reporting, RFS compliance, low-carbon fuel programs in California or Oregon), the feedstock claim on the bill of lading is more informative than the headline percentage.
Which blend should you use?
The honest answer is that for most people the choice is made for them — by the pump that's closest, by the fleet contract, or by the OEM warranty.
If you do have a choice:
- Run B5 or lower if you're driving an older diesel, a marine engine you don't want to risk, or anything stored seasonally. It's the lowest-friction option and works in essentially every diesel engine.
- Run B20 if your engine is on the OEM approval list (most post-2007 on-highway diesels are), you fuel up regularly, and you want the bulk of biodiesel's emissions benefit without the handling complications of B100.
- Run B100 only if your engine is specifically approved for it and you're set up to manage cold-weather operation, oxidation, and shorter storage windows. This is mostly fleet territory.
Cold weather: the part most people get wrong
Biodiesel gels at higher temperatures than petroleum diesel. The cloud point of B100 is typically in the 30-40°F range; soy-derived B100 cloud point is often quoted around 32°F. Petroleum diesel #2 clouds in the -10 to 10°F range, depending on the refinery batch.
That cold-weather penalty drops sharply with dilution:
- B5: cold behavior is essentially unchanged from base diesel.
- B20: cloud point typically 2-10°F warmer than the base diesel. Most fleets in cold climates handle this with the same winterizing additives they'd use anyway, plus a switch to a B11 or B5 blend during the coldest weeks.
- B100: requires winterization. Cold-flow improver additives, tank heaters, and seasonal switches to a low-blend or pure petroleum diesel are standard practice.
How blends are labeled at the pump
Federal Trade Commission rules require pumps selling biodiesel above B5 to display the blend level, with specific format requirements: a label reading "BIODIESEL BLEND — Contains 6 to 20 percent biodiesel" for B20, or "BIODIESEL — Contains 100 percent biodiesel" for B100. B5 and lower can be sold without a separate label, which is why most diesel pumps in the country technically dispense biodiesel without advertising it.
If a pump's label is missing or ambiguous, ask the station manager which ASTM spec the fuel meets. D975, D7467, or D6751 is the answer that matters.