That convertor used compressed natural gas though. But, apparently LPG weighs around 0.51KG per litre - https://www.elgas.com.au/blog/389-lpg-conversions-kg-litres-mj-kwh-and-m3/ - I'm not sure of the difference between LPG and natural gas. Although, that relation makes more sense, but still that would be close to R20 per litre anyway. So it's not really a saving as such vs petrol.
Blegh. I studied economics, none of this science nonsense.
"An amount of 8/9/10 Kg. CNG is stored in 40/50/60 litre size cylinder respectively."