The term lead sled has referred to two different types of cars. It was first used in the early decades of the 1900s to describe certain vehicles that had been heavily repaired. At the time, lead was a common filler material used to repair body damage. This sometimes resulted in vehicles that were literally full of lead, hence the somewhat derogatory term. The phrase was later used in reference to the large, heavy cars that became popular in the United States after the Second World War.
Lead was an early body filler used by both automotive factories and repair garages. As a soft, pliable metal with a low melting temperature and high resistance to corrosion, it was a popular filler material until various polymer resins were developed for the task. Lead was typically applied by cleaning the damaged area, applying a tinning substance, and then melting lead into the void. When applied properly, the lead would often be well fixed in place, though poor preparation of the work area could result in the lead simply falling out later on.
Since most body work involved the application of lead, and some factories even used it to seal body lines, not all lead work necessarily resulted in the negative connotation. The term lead sled was typically saved for vehicles that had undergone substantial repairs or modifications. It is still possible to do lead body work today, and car enthusiasts may sometimes use this old technique for more authentic results.
In the period after the Second World War, products made of polymer resins became more popular as auto body fillers. Larger, heavier vehicles also became more widespread. This resulted in a class of full sized cars known as lead sleds. These lead sleds were typically heavy and stylish, with more importance on looks than either speed or performance. The Mercury Eight, the first named vehicle produced by the Mercury auto company, was one such lead sled in the postwar period.
Lead sled can also refer specifically to vehicles such as the Mercury Eight that have been modified. Many lead sleds had their suspensions lowered and underwent other modifications. Such modifications include having various moldings and trim pieces removed and filled in.
The term lead sled is also used in the aviation industry. It refers to a number of different planes that had poor power-to-weight ratios. Some of the heavy planes to earn the nickname included jets like the F3H Demon and turbojets such as the F-84 Thunderjet.