Though modern cars don't use leaded gas, there was a time when a motorist could choose between leaded gas, unleaded gas and diesel fuel. Both filling stations and car manufacturers were interested in assuring that the right type of fuel was used for each engine. At filling stations, unleaded gas nozzles are smaller than those that were used for leaded gas, which were in turn smaller than the nozzles used for diesel. Car manufacturers made sure no one would insert a large nozzle into the tank by building in a plate under the gas cap that blocks the insertion of anything larger than an unleaded gas nozzle.
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