Every cargo tank that hauls fuel on a public road runs on a fixed test clock written into 49 CFR 180.407. For a standard DOT-406 or MC-306 gasoline tank the core of it is short: external visual and leakage test every 12 months, internal visual and pressure test every 5 years, and a thickness test every 2 years only if the tank is unlined and carrying something corrosive to it. Each test has a letter code, each has a hard deadline, and each has to be signed off by someone holding a DOT registration number that starts with the letters CT.
The six letters on the side of your tank
Walk around any bobtail or transport and look near the specification plate. You will find a stenciled or plated marking that reads something like 10-25 V, K. That is the month and year the tank last passed a test, followed by the codes for which tests it passed. Six codes exist, and every fuel hauler should be able to say them from memory.
- V, external visual inspection. Someone walks the tank, checks the shell, the piping, the emergency valves, the placard holders, the bolts, and the markings themselves.
- I, internal visual inspection. The tank is emptied, cleaned, ventilated, and entered. This is confined-space work.
- K, leakage test. The tank and its internal valves are brought up to a test pressure set by the rule and held while the tester watches for a pressure drop.
- P, pressure test. A hydrostatic or pneumatic test of the shell at a multiple of the tank's maximum allowable working pressure.
- T, thickness test. Ultrasonic readings of the shell and heads, compared against a minimum thickness table in the rule.
- L, lining inspection. Only applies to lined tanks in corrosive service.
A tank that passes only V and K in a given year gets marked with V and K. When the five-year internal and pressure tests come due, the marking is updated again. The date on the tank is the first thing a roadside inspector looks at, and it is the first thing a buyer looks at when you sell a truck. Both of them can read a stale date from ten feet away.
What a DOT-406 fuel tank actually owes
Here is the schedule for the tank most jobbers run, a DOT-406 or its older MC-306 cousin, hauling gasoline and diesel out of a bulk plant or off the rack.
Every 12 months: external visual inspection (V) and leakage test (K). These two run together at most facilities because the truck is already on the rack.
Every 5 years: internal visual inspection (I) and pressure test (P). The tank has to come out of service, get cleaned, and get entered. Budget a couple of days, not a couple of hours.
Every 2 years: thickness test (T), but only for an unlined tank carrying a lading that is corrosive to the tank. And here is the part that catches people: if an ultrasonic reading comes back below the minimum thickness in Table I of 180.407, the tank does not move to a shorter test interval. It comes out of hazardous materials service and stays out until the shell is repaired or the tank is replaced. There is no version of that rule where an under-minimum shell keeps hauling fuel on a tighter schedule. A failing reading is a stop, not a warning.
Corrosive service also collapses the other intervals. A cargo tank hauling a lading corrosive to the tank moves the internal visual and the pressure test from five years to one year. That is why a jobber who runs both fuel and a chemical side needs to know exactly what each trailer is dedicated to.
The thickness test question gasoline haulers get wrong
Gasoline and diesel are not corrosive to an aluminum DOT-406 shell, so a straight fuel bobtail generally does not owe a T test at all. Plenty of jobbers assume they do, pay for one, and never ask why. The reverse mistake is worse. If a tank has ever been swung into a service the owner considers corrosive to the shell, the clock starts and the burden of deciding sits with the tank's owner, not with the test facility.
That decision belongs in writing. Ask your inspector to note the service classification on the report, and keep it with the tank file. When you eventually sell the truck, that one line saves an argument. Anyone reading a deal on a jobber business will pull the cargo tank files before they pull the tax returns.
What a failed leakage test really costs
The annual leakage test is the cheap one. Two or three hundred dollars, an hour or two on the rack. The failure is expensive.
Take a 4,000-gallon bobtail running three drops a day, so roughly 12,000 gallons moved. At 10 cents a gallon of delivered margin that truck earns about $1,200 a day of gross margin. Now the leakage test fails because an internal emergency valve will not seat. The tank goes out of service, because a cargo tank that fails a required test may not be filled and offered for transportation until it is repaired and retested.
Six working days later the valve is replaced, the tank is retested, and the truck rolls. The bill: about $7,200 in lost delivered margin, a valve and labor somewhere in the $1,800 to $3,500 range depending on whether the tank had to be cleaned and entered, a second test fee, and the driver's wages while he sat. Call it north of $10,000 before you count the two accounts that got their weekend load from somebody else's truck. This is the kind of number that never shows up as a line item and quietly eats a quarter, which is the whole point of watching where fuel margin leaks.
The fix is not heroic. A driver's pre-trip that actually looks at the internal valve, an annual test scheduled in your slow month instead of the week before Memorial Day, and a spare truck or a standing rental agreement. If you run one bobtail and one transport, know which jobs each one can cover. The two trucks are not interchangeable, and finding that out during an outage is a bad time to learn it.
The CT number is what makes the paperwork real
Anybody who inspects, tests, repairs, or certifies a DOT cargo tank has to be registered with the U.S. Department of Transportation under 49 CFR Part 107, Subpart F. Registration produces a number that begins with the letters CT. No CT number, no valid test, no matter how good the mechanic is.
The rule also splits the work. Registered inspectors handle the visual and thickness work. Registered testers handle the pressure and leakage tests. Many shops hold both. When you book the job, ask for the CT number up front and check that it is current. It costs one phone call and it is the single most common gap that turns a routine audit into a bad afternoon.
What you keep and where
The tank carries the dated test marking. The report carries everything that proves it: the tank's serial and specification, the date, the test performed, the results, the name of the registered inspector or tester, and the CT registration number. The owner of the cargo tank holds those reports, and the rule says to keep each one until the next test of the same type is done. In practice, keep them for the life of the tank. They travel with the truck when it sells.
The leakage test and the vapor tightness test are cousins
Gasoline cargo tanks with a vapor collection system usually also owe an annual vapor tightness test under EPA rules, run to EPA Method 27. The DOT leakage test under 180.407 may be satisfied by a Method 27 test for tanks in petroleum distillate service equipped for vapor recovery, which is why one visit to the rack often covers both obligations. Confirm with your test facility rather than assuming, because the documentation has to name the right test to satisfy the right agency. Two tests, one truck, two sets of paperwork.
Propane runs a different clock
If you also run an LP-gas side of the business, do not carry the 406 schedule over to your MC-331 bobtails. Pressure vessels have their own intervals and their own inspection requirements inside 180.407, including provisions tied to the tank's original construction. Same regulation, different table row. The single worst compliance habit in a mixed fleet is one calendar entry that says "annual truck stuff."
Put the dates somewhere that yells
The cargo tank clock is one of maybe six recurring dates a small fuel operation has to hold. Registration, the annual DOT vehicle inspection under 49 CFR 396.17, the driver's hazmat endorsement, the tank tests, the vapor tightness test, and insurance. Miss any of them and the truck is a lawn ornament. A whiteboard works. A shared calendar works. In our own software at FastDragon we hang the test dates off the truck record so the dispatch screen shows a warning weeks before the marking expires, because nobody remembers a five-year interval on their own. Whatever you use, the test is simple: can a dispatcher at 5 a.m. see that the bobtail is legal today? If the answer lives in a filing cabinet, the answer is no.
The tests themselves are the easy part. Every year, one day on the rack, a few hundred dollars. It is the forgotten interval that costs real money, and the number that pays for the whole compliance program never shows up on the P&L at all. It shows up as the load you delivered on time when the other guy could not. That is one of the numbers that actually runs a fuel business, even though nobody can put a column on it.
Questions people ask
Is the annual DOT vehicle inspection the same as the cargo tank test?
No. The annual vehicle inspection under 49 CFR 396.17 covers brakes, lights, steering, tires, and coupling devices on the truck and trailer. The cargo tank tests under 180.407 cover the tank itself and its valves. Both are annual, both are required, and they are often done at different shops on different days. Two stickers, two files.
How much grace do I get after the marking date expires?
None worth relying on. The rule works in whole months, so a tank marked 10-25 for a 12-month test is due by the end of October 2026. Once the interval lapses the tank may not be filled and offered for transportation. There is a narrow provision allowing an expired tank to be moved empty for repair or testing under specific conditions, so talk to your inspector before you move it. Do not load it.
Can my own shop do the external visual inspection?
Only if the person performing it is a registered inspector under Part 107 Subpart F with a current CT number, and meets the qualification requirements in 49 CFR 180.409. Some larger jobbers register their own maintenance lead and do the annual visuals in house. It is worth the paperwork if you run more than a handful of trucks. Below that, hiring it out is cheaper.
I am buying a used bobtail. What should I ask for?
The specification plate photo, the current test marking photo, and the last five years of test and inspection reports with legible CT numbers. Then ask specifically whether the tank has ever been in service the seller classified as corrosive to the tank, and whether any thickness reading has ever come back below the Table I minimum. If one has, that tank was required to leave hazardous materials service until the shell was repaired or the tank was replaced, so ask to see the repair documentation and the passing retest rather than taking a verbal answer.
What happens to the test clock if the tank gets repaired?
Repairs that affect the tank shell or its pressure boundary generally require a pressure test and an inspection before the tank returns to service, performed by a registered facility, and the repair itself has to be documented and retained. A welded repair on the shell is a different event than swapping a hose or a fitting. Ask the repair shop which category the work falls into before they start cutting.