Abstract: A vehicle fuel tank construction which includes a body portion defining a reservoir for a supply of diesel fuel and having a fuel inlet which is provided with a threaded portion or adaptor to which a seal cap or closure is detachably connected. The sealing cap and fuel inlet or adaptor are provided with complementary mating surfaces or threads which will function as a conscience indicator to alert a filling station attendant or person that the tank requires diesel fuel. This is attained by providing the tank cap with complementary thread portion requiring the attendant to remove the cap in a manner opposite to that required of gas tanks used for gasoline. Also, the tank cap may be made in a shape other than round.