Cars are converted to propane or Liquefied Petroleum Gas (LPG) generally because their operating costs are much lower than for gasoline. Nowadays, automobile conversions are much more popular in Europe than in North America. Conversions were quite popular in Canada in the 1980s because of government incentive programs. In North America, the best candidates are vehicles in fleets driven long distances because the lower operating cost makes a faster payback in recovering the capital cost of conversion. The main concern in this type of conversion is cost and performance is a minor issue.
Although propane has a higher energy content (J/kg or BTU/lb) than gasoline, it has a lower density (kg/litre or lb/gallon). Because of its lower density, a car running on propane will have a higher fuel consumption than one running on gasoline but its higher energy content will compensate somewhat for the higher fuel consumption. When the fuel price is factored into consumption, the driving cost ($/km or $/mile), propane is significantly cheaper to use.
An additional benefit of propane is that, as a cleaner burning fuel, fleet vehicles last much longer between overhauls. Propane does not produce carbon deposits in the combustion chamber no matter how long it is used. Without the carbon deposits, engines do not require higher octane fuels to compensate for the gradually increasing compression ratio or the hot spots that cause preignition. There is no carbon to blow by the rings to contaminate the oil, which allows the oil to remain clean. Engine oil at 10,000 km will look as clean as newly changed oil. The main problem with the oil is that extended distances between oil changes cause the lighter parts of the oil to boil off and the oil gradually becomes more viscous. Modern engine oils are formulated for longer oil change intervals (compared with engines of the 60s and 70s) so it's wise to follow the oil change intervals specified in your owners manual. For extended oil change intervals, it is useful to do Used Oil Analysis to verify that the extended interval is safe.
A good place to start research for a propane conversion is Jay Storer's book: "Economy or Performance Propane Fuel Conversions for Automotive Engines (1986-01-24)". It was published by S-A Design (ISBN 0-931472-12-1) and is still available on Amazon. A few book sellers still seem have it stock however. Another useful book to read is Larry W. Carley's Propane Conversion of Cars, Trucks and Rvs. It is published by TAB Books (ISBN 0-8306-3103-8) and this book is also available on Amazon. You may also have luck finding these books in your local library.
An excellent source of information is Franz Hofmann's publication: Diagnostic Guide to Alternative Fuels. Besides great general information about LPG-fuelled engines, it also has a lot of LPG engine-building and repair advice. His very reasonably priced book contains the most current and comprehensive information about North American alternative fuel systems available anywhere. To order a copy of the Diagnotic Guilde, contact Franz Hofmann directly.
Cars converted to propane generally suffered a performance loss in the bad old carbureted days. The problem was that propane was metered (or fumigated) into the engine in a gaseous form. The fuel gas displaced air that the engine was drawing which caused a slight loss in volumetric efficiency. Gasoline is metered into the engine as an atomized liquid. The density of a liquid is much greater that that of a gas and so the volumetric efficiency loss compared to pure air is negligible. The other contributor to lower volumetric efficiency is the somewhat restrictive mixer.