Comment by ErroneousBosh

It depends how you do it. The conversion on my elderly Range Rover is not as efficient, being an old-fashioned "single point" system - this has a thing a bit like a cooker ring attached to the throttle, a stepper motor controlled valve driven by a third lambda sensor, and a thing to disconnect the petrol injectors when it switches to gas.

You can get ones with a complete set of LPG injectors and an ECU that takes its timing from the petrol injectors, and these are incredibly efficient. They're a bit harder to install (you need to drill holes in the intake manifold, it's a faff) but the engine can be mapped for even more power than on petrol and a tiny amount of pollution.

There's still a lot of CO2 and water vapour, but as previously discussed you're burning all this shit anyway so you might as well extract useful work from it.

The time to have done this was 25 or 30 years ago, when it was ridiculously cheap to buy gas and there were a lot of old-fashioned carburettor-fed cars that were incredibly badly polluting.