A truck tractor typically carries its fuel on the outside: in tanks hung from the frame, beneath the cab, where the next lane begins. The federal crash test that asks whether a fuel system holds in a collision is written for cars and light trucks. The heavy tractor answers to a different rule, written for the company that operates it, and that rule measures the tank by itself. What it asks, what it leaves unasked, and what diesel does once it is out are the questions here.
Where the fuel rides. On the road they are called saddle tanks. The federal rule calls them side-mounted, and it defines the thing by where it sits: on a truck tractor, a side-mounted tank is one that “extends outboard of the vehicle frame and outside of the plan view outline of the cab” (49 CFR 393.67). The companion rule covers location, attachment and fuel lines. “No part of the system extends beyond the widest part of the vehicle”; “No part of a fuel tank is forward of the front axle of a power unit”; “Each fuel tank must be securely attached to the motor vehicle in a workmanlike manner”; and “Diesel fuel crossover, return, and withdrawal lines which extend below the bottom of the tank or sump must be protected against damage from impact” (49 CFR 393.65). Read what is protected. The lines are, and in the tank rule so are the drains: “Drains or other bottom fittings must be protected against damage from impact.” Neither section, as posted, asks for a shield around the tank itself. And both are rules for the company that runs the truck: “No motor carrier may operate a commercial motor vehicle, or cause or permit such vehicle to be operated, unless it is equipped in accordance with the requirements and specifications of this part” (49 CFR 393.1).
The test a car gets and a tractor does not. The federal fuel-system standard for motor vehicles states its purposes in its first lines, and the first is “to reduce deaths and injuries occurring from fires that result from fuel spillage during and after motor vehicle crashes.” It works on the whole vehicle: after its crash tests the vehicle is turned over and what leaks is measured, the part of the procedure The Roof and the Tank walks through. But read the line that says what it covers. The standard “applies to passenger cars, and to multipurpose passenger vehicles, trucks and buses that have a GVWR of 4,536 kg or less and use fuel with a boiling point above 0 °C,” and to school buses above that weight (49 CFR 571.301). The initials are the gross vehicle weight rating, and 4,536 kilograms is 10,000 pounds. The federal ceiling on the gross weight of a loaded tractor-trailer on the Interstate is 80,000 pounds (23 CFR 658.17), eight times that line. A tractor rated above 10,000 pounds is outside the standard. The truck rules say the same thing from the other side: “Motor vehicles that meet the fuel system integrity requirements of 49 CFR 571.301 are exempt from the requirements of this subpart, as they apply to the vehicle’s fueling system” (393.67). There are two sets of rules. One crashes the vehicle. The other, written for the carrier, does not.
The tank’s own tests. What the truck rule measures is the tank, off the truck. Its words are careful: each tank “must be capable of passing” the tests, and the tests “are a measure of performance only.” A tank that holds more than 25 gallons must have a venting system that, “in the event the tank is subjected to fire, will prevent internal tank pressure from rupturing the tank’s body, seams, or bottom opening”; in the test the tank is filled three-fourths full, turned upside down and wrapped in flame, and the vent “must activate before the internal pressure in the tank exceeds 50 pounds per square inch, gauge.” In the leakage test the capped tank is turned “through an angle of 150° in any direction about any axis from its normal position,” and it may not “leak more than a total of one ounce by weight of fuel per minute.” A side-mounted tank must be capable of passing two more, and they are the heart of the rule. “Fill the tank with a quantity of water having a weight equal to the weight of the maximum fuel load of the tank and drop the tank 30 feet onto an unyielding surface so that it lands squarely on one corner.” Then it is dropped 10 feet “so that it lands squarely on its fill-pipe.” After each drop the measure is the same ounce a minute, of water this time. The rule does not require that any particular tank be dropped: “Alternative procedures which assure that equipment meets the required performance criteria may be used.” The maker certifies conformity on the tank itself, which must be “legibly and permanently marked by the manufacturer” with, among other things, “The month and year of manufacture” and, for a tank that conforms to the side-mounted rules, the words “Meets all FMCSA side-mounted tank requirements” (393.67). The date matters, because a tank “manufactured on or after January 1, 1973,” must conform to every rule in the section.
It is a real test, and a hard one for what it measures. A thing dropped 30 feet lands at about 30 miles an hour, and from 10 feet at about 17. But the only weight behind the blow is the tank’s own, full. It is a test of a tank against a floor. It is not a test of a tank caught between a frame rail and the front of another vehicle, where the weight behind the blow is the vehicle’s (Outweighed), and it is a test of the tank as built, not of one that has spent years on the road.
What the investigators wrote. In 2016 a bus and a truck tractor collided at a rural intersection in Florida, and the National Transportation Safety Board investigated. Its report put the fire on the truck’s fuel: “The truck-tractor’s right-side-mounted fuel tank was compromised during the collision sequence, and a postcrash fire ensued.” The explanation it gave was about where the tank was and what it was made of: “The location of the tank beneath the truck cab and outside the frame rail made it vulnerable to impact forces and structural failure,” and “the lack of protective shielding and the tank’s aluminum construction increased its susceptibility to being breached.” The Board counted “heavy truck fuel tank integrity” among the safety issues of the crash, and it named the rupture of the tank, “leading to a fast-spreading postcrash fire,” as something that contributed to the severity of the injuries (NTSB/HAR-17/05). It then asked the federal highway safety agency to work with the truck safety agency and with SAE International “to improve truck-tractor side-mounted fuel tank crashworthiness to prevent catastrophic tank ruptures and limit postcollision fuel spillage, and develop and promulgate an updated standard.” The agency answered in April 2018 that the recommendation was “under consideration as a new item of work” for an SAE International committee (NHTSA letter of April 13, 2018, on Safety Recommendation H-17-62). The tank rule as it is posted today is the one described above. Its measure of a side-mounted tank is still a drop.
One caution about that report, because it matters in a courtroom. By federal statute, “No part of a report of the Board, related to an accident or an investigation of an accident, may be admitted into evidence or used in a civil action for damages resulting from a matter mentioned in the report” (49 U.S.C. 1154). The Board’s own rules draw the other half of the line, and it concerns different documents, the investigators’ factual reports: “The Board does not object to, and there is no statutory bar to, admission in litigation of factual accident reports” (49 CFR 835.2). So the Board’s conclusions, the ones quoted on this page, are a map and not evidence. They tell an engineer what to measure on the next tank.
The fuel. Diesel is not gasoline, and the difference is real. NOAA’s chemical database gives gasoline a flash point of 36 degrees below zero Fahrenheit and says it “Can be ignited under almost all ambient temperature conditions” (NOAA, CAMEO Chemicals, Gasoline). It gives diesel fuel oil a flash point of 125 degrees Fahrenheit and says it “Must be moderately heated or exposed to relatively high ambient temperatures before ignition can occur” (CAMEO Chemicals, Fuel Oil, Diesel). The international safety card for diesel fuel puts the same point at 52 degrees Celsius and adds the sentence that matters at a crash: “Above 52°C explosive vapour/air mixtures may be formed” (ILO and WHO, ICSC 1561). A crash is where that heat is found: an engine and an exhaust at working temperature, brakes, steel dragged along pavement, sometimes a vehicle that is already burning. The Florida crash is one instance. The breach of the diesel tank, in the Board’s words, “ignited a fire.” And once a tank is in the fire, the federal Emergency Response Guidebook, in the guide reprinted on NOAA’s datasheet, tells responders what to listen for: “Withdraw immediately in case of rising sound from venting safety devices or discoloration of tank,” and “ALWAYS stay away from tanks in direct contact with flames.”
What fire does. Fire is the page on the body. It is two injuries and not one: burns over a large area of skin, which drive the body into shock, and the injury to the airway and the lungs from breathing superheated gas and smoke. Both work on a clock, and the people in a crushed car beside a burning tank may not have the minutes. That is why the fire is treated as a second event. The first question is whether the impact alone was survivable. If it was, the fire is a separate harm, and it has causes of its own.
What the law asks. Three questions, and it helps to keep them apart. The first is who caused the crash. That is negligence, it may be the truck driver’s, the other driver’s or both, and Texas divides it by percentage: a claimant recovers nothing “if his percentage of responsibility is greater than 50 percent” (Civil Practice and Remedies Code Section 33.001), and below that line the award is reduced by the claimant’s share (Section 33.012; Comparative Fault). The second is why there was a fire. Texas has the jury set a percentage of responsibility for each claimant, each defendant, each settling person and each designated third party, for causing or contributing to the harm, “whether by negligent act or omission, by any defective or unreasonably dangerous product,” or by other conduct that violates a legal standard (Section 33.003). And in 1979 its Supreme Court quoted a federal appeals court that had, it said, “succinctly stated” the point: a person hurt by a mechanical defect in a vehicle “should be protected under the doctrine of strict liability even though the defect was not the cause of the collision which precipitated the injury” (Turner v. General Motors). The case the Texas court was quoting was not about a car. It was about a truck tractor that jackknifed and struck an overpass support, and in that court’s words, “the fuel tank ruptured and caught fire” (Huff v. White Motor Corp., decided under Indiana law). That doctrine, what a design claim has to prove, and what meeting a federal standard does and does not do for a manufacturer in a Texas court are in Crashworthiness. The third is what the carrier did with the tank it had. The federal rules put its condition on the company that runs the truck, which “must systematically inspect, repair, and maintain” its vehicles: the tank “must be securely attached,” the lines below it “must be protected against damage from impact” (393.65), and “Parts and accessories shall be in safe and proper operating condition at all times” (49 CFR 396.3). The truck safety agency’s guidance adds that “A replacement/repaired tank must meet the applicable standards in § 393.67” (FMCSA guidance). A strap worn through, a tank patched, a cap that does not seal, a replacement tank without the marking: each is a fact about maintenance, and a broken safety rule is evidence of the standard of care, not a verdict. What a violation does and does not prove is the subject of Negligence Per Se (see also Inspection and the page on trucking company liability).
The evidence. A fire destroys a great deal, and what is left is still evidence. The tank: where it opened, at a seam, a corner, the fill pipe or a fitting, and what struck it. Its marking, with the month and year it was made and the maker’s certification. The straps and brackets that held it, the cap, the crossover line and whatever guarded it. The burn pattern on both vehicles and the fire department’s report of the run, which can show where the fire began and how fast it moved. The engine computer and the electronic log (truck black box and ELD data), and the carrier’s inspection and maintenance file. And the tractor itself, which can be repaired or sold for scrap before anyone has measured it, unless someone asks in writing that it be kept (Spoliation).
Why it matters. For the people who lived through the impact, the fire was the event, and it had its own causes: where the fuel rode, what stood between it and the blow, and what the tank had ever been asked to survive. The rule answers that last question exactly. A fall of thirty feet onto one corner, with nothing behind it but its own weight.
49 CFR § 393.1 (paragraph (c)), § 393.65 (paragraphs (b), (c) and (f)), § 393.67 (paragraphs (a)(1), (a)(7), (b)(2), (c)(4), (c)(8), (d), (e) and (f)), § 571.301 (S2 and S3) and § 396.3 (paragraph (a)), 23 CFR § 658.17 (paragraphs (a) and (b)) and 49 CFR § 835.2, and 49 U.S.C. § 1154 (subsection (b)), as posted by Cornell LII; National Transportation Safety Board, Agricultural Labor Bus and Truck-Tractor Collision at US-98-SR-363 Intersection Near St. Marks, Florida, July 2, 2016, Highway Accident Report NTSB/HAR-17/05 (adopted November 28, 2017; the abstract and the executive summary); National Highway Traffic Safety Administration, letter to the Chairman of the National Transportation Safety Board, April 13, 2018 (the text of Safety Recommendation H-17-62 is quoted as it appears in that letter); Federal Motor Carrier Safety Administration, guidance on § 393.67, What standards under the FMCSRs must be met when a liquid fuel tank is repaired or replaced?; NOAA Office of Response and Restoration, CAMEO Chemicals, Fuel Oil [Diesel] and Gasoline (flash points, the NFPA 704 flammability descriptions, and for diesel the Emergency Response Guidebook, Guide 128); International Labour Organization and World Health Organization, International Chemical Safety Card 1561, Diesel Fuel No. 2 (October 2004); Turner v. General Motors Corp., 584 S.W.2d 844 (Tex. 1979), quoting Huff v. White Motor Corp., 565 F.2d 104 (7th Cir. 1977); Tex. Civ. Prac. & Rem. Code § 33.001, § 33.003 and § 33.012, as posted on FindLaw, current as of January 1, 2026. The arithmetic: a body falling 30 feet with no air resistance lands at about 44 feet a second, which is 30 miles an hour, and from 10 feet at about 25 feet a second, which is 17; 4,536 kilograms is 10,000 pounds. This is general information, not legal advice. Text and figures as published at publication.
