Plug-in hybrids tend to have bigger batteries and smaller engines than conventional hybrids and are designed to be charged regularly. If they're not charged, they're woefully inefficient as they're effectively an underpowered car carrying an awful lot of weight. It's also worth noting that, during the old NEDC fuel economy tests, plug-in hybrids would start with a fully-charged battery. This would result in a large disparity between the official and real-life figures if owners didn't charge them regularly. Our report was based on thousands of fuel economy figures provided by readers: www.honestjohn.co.uk/real-mpg/