Aredwood: LPG mainly reduces carbon emissions from car engines due to the displacement effect. As the gaseous LPG takes up room in the combustion chambers that would otherwise have air in it. The engine is effectively a smaller capacity now. And as a result, it also has a lower power output. Although since LPG has a higher octane rating, the engine could sometimes be retuned to get the power back.
Because of the above, cars with larger engines were the most popular for converting to LPG. Almost every taxi driver used to own an LPG Holden Commodore or Ford Falcon. But they had a 3.8L V6 and a 4L straight 6 engine respectively. So still plenty of horsepower for the job. But taxi drivers mostly use hybrid cars now. Especially after Toyota started making a hybrid version of the Camry. Which shows that LPG is still more fuel hungry compared to hybrids.
LPG would be far better suited to long distance trucking, rail transportation (where electric rail doesn't stack up) and shipping. As it is a relatively simple modification to add LPG supplementation onto diesel engines, to reduce diesel usage. And if the LPG tank runs out, the engine keeps going on diesel. Same thing can also be done with Natural gas. And some US rail companies are doing exactly that. To save money on diesel.