Dunno the actual volume, bu if the brochure says 66L, then knowing VW that will be the ‘usable volume’. Total volume will be a bit bigger because there needs to be volume to spare for when the fuel expands in the hottest climates. Usable volume also doesn’t necessarily include a few litres that - depending on which way the vehicle is leaning, may not be able to be scavenged by the lift pump.