Which DOT cargo tank specification is a non-pressure tank with a maximum allowable working pressure (MAWP) of only 2.65 to 4 psig, commonly used to haul gasoline and other petroleum products?

49 CFR 178.346 (DOT 406 cargo tank specification)

Recognizing that most gasoline tankers on the road are low-pressure DOT 406 tanks, not high-pressure vessels, helps explain why they are oval and non-circular in cross-section and why they are never used for pressurized gases like propane, which require an entirely different tank design.