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The catalyst formulation and loading used will vary greatly from one (gasoline) vehicle to another, based on the engine's control strategy, the relevant emissions control legislation, the engine's size and where the catalyst is positioned on the car. The worldwide average platinum group metal content is around 4-5 grams per car, but the range is very wide - from 1 g on microcars to 15 g or more for really big, powerful vehicles. Currently (2007), most gasoline-fuelled vehicles use a combination of palladium and rhodium as the catalytic metals, but a number also use platinum. Something like five times as much palladium as rhodium is estimated to be used for autocatalyst applications.

By contrast, in diesel vehicles, although the catalyst loading and catalyst size vary greatly, the metal formulation varies little, being mainly based on platinum. Platinum is easily the most efficient metal for reducing emissions from the oxygen-rich exhaust of the diesel engine, particularly in the presence of sulfur.

Answer posted 26 June 2007