What is the difference between an MC-306 and a DOT-406?
Both are cargo tank specifications for gasoline, diesel and similar liquids. Under 49 CFR 180.405, no cargo tank may be marked or certified to the MC-306 specification after August 31, 1995, with a limited recertification exception covered below. New builds are therefore DOT-406.
The DOT-406 is defined in 49 CFR 178.346-1. Its maximum allowable working pressure (MAWP) must be no lower than 2.65 psig and no higher than 4 psig, and it must also meet the general design rules in 178.345. Older tanks are not banned. Part 180 lets a tank made to the older specification stay in use if it was marked or certified before September 1, 1995 and keeps meeting the periodic tests.
So for a broker or supplier the spec letters tell you roughly how old the tank is. Whether it can haul your load depends on whether this tank is current on its tests today. (For how much a tank holds and how compartments work, see how many gallons a fuel tanker holds.)
Can an old MC-306 still haul my fuel?
Yes, if it was certified before the cut-over and keeps passing its periodic tests. Nothing in the rule retires a 306 on age alone.
There is one wrinkle to look for. A tank originally built to MC-306 may be recertified to that original specification, but only if records verify how it was built and modified, a Design Certifying Engineer or Registered Inspector verifies it conforms, and it meets all the periodic tests. In that case the specification plate must display the date the tank was originally certified. A 306 plate with only a recent recertification date is a question to put to the carrier.
What do the spec plate and markings tell you?
The plate shows the specification and original test date, and the test markings show what was inspected and when. 49 CFR 178.345-14 requires a corrosion-resistant nameplate on the left side of the vehicle near the front of the tank, in a place readily accessible for inspection. Among other fields it lists the DOT specification number, original test date (month and year), MAWP, test pressure and nominal capacity.
Separately, 49 CFR 180.415 requires each tank that completes its tests to be marked with the month and year and the type of test, in characters at least 1.25 inches high, near the specification plate or anywhere on the front head. The abbreviations:
| Marking | Test or inspection |
|---|---|
| V | External visual inspection and test |
| I | Internal visual inspection |
| P | Pressure test |
| K | Leakage test |
| K-EPA27 | Leakage test under the EPA Method 27 route, after October 1, 2004 |
| L | Lining inspection |
| T | Thickness test |
You can read these off the truck, or from a photo the driver or carrier sends. If the markings aren’t legible, sort that out before the tank loads.
Which periodic tests apply to a gasoline or diesel tank?
49 CFR 180.407 sets the schedule in its table. This is the buyer-side reading for the common case, an uninsulated, unlined tank:
| Test | Interval | What it tells you | What to ask the carrier |
|---|---|---|---|
| External visual (V) | 1 year (6 months for vacuum-loaded tanks with full-opening rear heads) | Shell, piping, valves and gaskets checked; emergency valves and remote shutoffs functioned | Date of the last V marking |
| Leakage (K) | 1 year | Product piping and self-closing stop valves hold pressure; test pressure at least 80% of marked MAWP, held at least 5 minutes | Date of the last K or K-EPA27 marking |
| Internal visual (I) | 5 years | Inside of shell and heads checked for corrosion, dents and weld defects | Date of the last I marking |
| Pressure (P) | 5 years | Tank wall holds a hydrostatic or pneumatic test pressure | Date of the last P marking |
| Thickness (T) | 2 years, only for unlined tanks carrying material corrosive to the tank | Wall has not thinned | Whether this row applies to the tank |
For a DOT-406 that carries petroleum distillate fuel and has vapor recovery equipment, 49 CFR 178.346-5 allows the leakage test to be done under 40 CFR 63.425(e) and marked under 180.415. The table is a general schedule, and some tank types and services have other intervals, so ask which row applies to this tank as well as for a date.
Under 180.407(a), a tank whose test or inspection “has become due, may not be filled and offered for transportation or transported until the test or inspection has been successfully completed.” A tank already filled before the due date is excepted.
How do you read the markings? A worked example
Say it is October 2026 and the carrier sends a photo of the front head of the trailer booked for your load. All numbers are illustrative.
| Marking on the tank | Interval | Due by | Verdict |
|---|---|---|---|
| 09-25 V | 1 year | September 2026 | Overdue: the external inspection came due last month |
| 03-26 K | 1 year | March 2027 | Current |
| 06-22 I, P | 5 years | June 2027 | Current |
Everything is current except the external visual inspection, and that alone means the tank should not be filled for your load (180.407(a)). Ask for the new V marking date, or a different tank, before dispatch. If there were no K marking at all, you could not tell whether the leakage test was ever done.
Some conditions require a test regardless of the calendar. Under 180.407(b), a tank that shows dents, cuts, gouges, corroded areas or leakage, or has been out of hazardous materials transportation service for a year or more, must be tested before further use. A tank that fails must be repaired and retested, or removed from hazardous materials service with its specification plate removed or covered.
What should go in the carrier file?
Ask for four things and keep them with the carrier’s other documents:
- The unit number or plate of the tank that will run your load, so the records match the truck that shows up.
- The date of each V, K, I and P marking, or the inspection records behind them.
- For an MC-306, the original certification date on the plate.
- Which test intervals apply to this tank.
Operating authority, insurance and driver endorsements are separate checks, covered in how to vet an external fuel hauling company. Being carded at a rack tells you a carrier can load there; it does not tell you the tank is current. See carded terminal for what that credential covers, and, where you can, ask that the tank’s unit number appear on the bill of lading so the load and the tank record stay tied together. To find carriers in the first place, see the fuel tanker load boards guide.
How Energy Connect handles it
Energy Connect records dispatch, pickup, transit and delivery on the load, shows pickup, delivery and verification times, and lets you attach the bill of lading and other delivery documents to the load.
The tank inspection dates above come from the carrier, so ask for them before the load is dispatched.