An older American vehicle (yank), which are known for being very fuel-inefficient, really large (tank) and often very uniquely designed. Most yanktanks were produced before the 1973 fuel crisis, but some are still produced today (Lincoln Town Car and Ford Crown Victoria for instance).
Some however think that the size of the engine of an yanktank should be given in cubic inches, and if it isn't it's not a real yanktank.
Fred: Scotty drives a real yanktank.
Mike: What does he drive then?
Fred: He's got a '64 Impala.
Mike: Oh, yeah, that's an yanktank for sure.