According to the U.S. Department of Energy, propane reduces greenhouse gas emissions by about 10% compared to modern gasoline and diesel engines, is equivalent to gasoline and diesel in terms of tailpipe emissions, and is non-toxic, non poisonous, and insoluble in water [but this is nothing compared to the potential for alcohol as a transportation fuel]. For high mileage vehicles, maintenance costs are reduced. Another advantage is increased resilience to the extent that your household increases the variety of fuels it can use. For a more detailed look at propane and benefits and obstacles to wider adoption nationally see this 2010 paper which points out that propane costs more than gasoline on an equivalent mileage basis.

Propane can be used in both small and medium engines when these engines are converted to run on propane. Large engines, such as for cars and trucks, can also be converted (see also, this source) with an upfront cost of $4,000 to $12,000 for a “light-duty vehicle” (source).