That depends entirely on where the gas comes from.
The reason the Government offered money for conversions is that a lot of the UK's own oil production has gas associated with it. The gas was treated as a waste product and was being flared off, resulting in CO2 emissions but without doing anyone any good.
To prevent these wasteful emissions of CO2, Government decided to encourage the oil companies to collect the gas and, by offering a lower rate of duty and help towards the cost of conversion, encourage its use in motor vehicles.
The thinking was that vehicles using LPG would not be using petrol/diesel, so the CO2 emissions from the petrol/diesel their owners did not burn in their cars would be saved. The same amount of CO2 would be emitted from the LPG whether it was flared off or used in a vehicle, so there was a significant net saving in CO2 emissions.
Of course there came a point where the sales of automotive LPG rose to equal the supply of oil-associated gas from the oil companies. At that point, the Government stopped offering subsidies for conversion but the duty advantages of LPG were retained.
Over the next few years, as North Sea oil production falls, the supply of oil-associated gas will also fall. That is why the duty advantage of LPG is gradually being eroded, to make sure that only the oil-associated gas is subsidised.
If the LPG doesn't come from oil-associated gas, it has to be manufactured from natural gas, and that obviously doesn't offer any saving at all in CO2 emissions. The saving arises only where gas would otherwise be flared off as a waste by-product of oil extraction.
Very good points.
