The mistake is that many think this is about MPG.
The real reason for this is more about the off cycle credits the MFGs earn. The fuel saving is there but negligible. But the credits earned by the MFGs are great.
The system works today mostly due to the computer abilities we have today. Killing odd the cylinders mechanically was not difficult but getting it so work seamlessly was difficult.
Todayís sensors and computers are light years ahead of what we had in the 80ís.
The off cycle credits are more about emissions that can not be measured in normal driving and the government awards credits base on the added technology.
Here is something that better explains what is going on today with some of the technology we are seeing used.
These credits are widespread in what is counted and how they are used. Everything from cooling seats, paints, cylinder drop, special glass etc can be counted.
So if something that makes little sense is being used that appears to not save much gas it may be more for emissions.
The Start Stop systems are a part of the Off Cycle program too. It is another thing seen as cutting emissions.
Automakers may market the as fuel saving as they may to a small degree but most of this is due to emissions.
4 cylinders polite less than 6 or 8 when not under power.