To calculate the autonomy of a car fuelled by CNG, it is necessary to know that 1 kg of CNG corresponds about 1.7 litres of petrol and that the coefficient of distance of CNG/petrol is 0.3 (therefore with a 70 litre CNG cylinder the distance covered is comparable to that covered with about 21 litres of petrol).
An example: let's suppose to have a 70 lt CNG cylinder; it allows therefore a distance comparable to about 21 lt of petrol (70 lt x 0,3); let's suppose that the car runs with petrol 10 km with a litre; to calculate the distance, we will have to use the following formula: 70 lt x 0.3 x 10 = (21 x 10) = 210 km of autonomy on natural gas.