These cars are extremely good on fuel if maintained properly, mostly the bad average isn't because of the vehicle but due to lack of expertise to troubleshoot minor issues. The chevy exclusive at our home has never given below 15 km/ litre and its more then a decade old now, still going strong.
Like in most vehicles, things like fuel filter, clogged injectors, wide improper sized tyres with cheap heavy rims, low quality air and oil filters, cng setups connected while not being used etc are the causes of bad fuel economy