Complex sentence: Since 2012 there has been a slight increase in the number of autogas-powered vehicles of around 14%, mainly due to the phasing-out of the tax incentives to buy diesel cars.
Split into simpler sentences: Since 2012 there has been a slight increase in the number of autogas-powered vehicles of around 14%. This increase is mainly due to the phasing-out of the tax incentives to buy diesel cars.