Rent machine with fuel

You have to charge the JCB rent along with diesel consumption. This is Composite supply. Hence, has to charge GST on both the values.

As per Section 15(2)(b) of CGST Act- ''any amount that the supplier is liable to pay in relation to such supply but which has been incurred by the recipient of the supply and not included in the price actually paid or payable for the goods or services or both;

GST will be charged on Rental Income+ Diesel Charges. Various Court Judgement are also there which says that diesel charges will be included while discharging GST amount.