Tough CAFE standards, even tougher economic conditions and high oil prices have conspired to lay the ultra-luxury Cadillac XLS and its V12 engine to rest. Never officially confirmed for production, there had been rumors that it was being worked on internally. At the same time, Cadillac's on-again, off-again DTS/STS replacement, though on hold, is still being considered.

The now-defunct XLS was expected to take cues from the even more extravagant Cadillac Sixteen concept car (pictured), which was shown in 2003. The XLS had been envisioned as the halo model to rally Cadillac's return to the top of the full-line luxury ranks. Now, with the project on the shelf and the DTS/STS replacement in a holding pattern, the brand is more or less left without a true range-topper.

The V12 engine that would have powered the XLS was expected to be based around two 3.6L V6 engines fused together. That combination would have proved good for an estimated 600hp (447kW) and 540lb-ft (730Nm) of torque. Cadillac was also expected to include direct injection and cylinder deactivation to make the most of the big engine's efficiency, warding off CAFE-related fears.

Regarding the DTS/STS replacement, the company is reportedly still in development, according to Edmunds. The car that will eventually take the top sedan spot, known internally as the DT7, will still ride on a RWD version of the Zeta platform, but won't come along until late 2011 or 2012 now. That will give the company enough time to run out the existing DTS and STS models, deplete any remaining stock, and leave a brief gap before the new car's introduction.