What mileage can I get from my car if I switch to methane?

This is calculated for each individual car from the size of its gas cylinders (their capacity in litres is stamped on the metal and is also specified on the identity plate attached to the vehicle by its manufacturer or a methane conversion workshop).
The total cylinder capacity (in litres) is then divided by four (more or less) to give the cubic-metre equivalent of the methane on board.
For example, an 80-litre cylinder holds the equivalent of about 20 cubic metres of methane.
Assuming an average of 14 km per cubic metre of methane, the car will do about 280 km on a full tank.