AFTER President McCain or Obama takes the oath of office in January, he may have a shiny new black limousine to go along with his not-so-new White House. General Motors is believed to be putting the final touches on a new First Car.

An analysis of unauthorized photographs taken while the car was being tested last summer on public roads suggests that the presidential ride will be a truck-based Cadillac. It will presumably replace the Cadillac that President Bush has used since 2005.

This new car will be a Caddy like no other. The photos by Chris Doane, a spy photographer who hunts big automotive game  future models that haven’t been publicly revealed  for magazines and Web sites, provide clues about how specialized presidential transportation has become since the first White House fleet was ordered for William Howard Taft in 1909. President Taft rode in a stock White steam car or a conventional Pierce-Arrow, but the next president will travel in a fortress-like vehicle that was mostly built from scratch.

The photographer noted that the limousine was being tested, possibly for comparison purposes, with a pair of GMC Topkick medium-duty trucks. The limousine seemed to be riding on the same 19.5-inch Goodyear Regional RHS tires as the trucks, indicating that it is far heavier than a civilian Cadillac  even the longest stretch limousines built with the G.M. division’s heavy-duty coachbuilder package. Indeed, it is believed that the limo is based on G.M.’s 2500 line of trucks, which includes an extra-heavy-duty version of the Suburban.