Gov't standards are one thing - the tests are anachronistic, and do not come close enough to represent real world driving.
CAFE MpG tests explain undergeared axles for decades, followed by smaller engines with less cylinders mated to 8- & 10- speed transmissions.
That said, my suspicion is GM went ahead with V8-V7-V6-V5-V4-V3-V2 mode because they don't want the engine to last TOO long.
That extra complexity is gonna bite them in the arse when Mexico's used car market hemorrhages into the southwestern US.