Is it a good idea to run a car on 3 cylinders? How long will the engine
>last doing this? I have noticed about a 30% decrease in fuel efficiency.
For a regular 4 cylinder engine, that's to be expected as with one less cylinder firing, you automatically lose 25 percent of the rated output, but it'll be a bit more due to the extra work the other three cylinders need to do to overcome the mechanical imbalances one non-firing cylinder induces.
I've noticed some car makers advertising they make their new car designs more 'efficient' by shutting down some cylinders, but I expect there is some special design feature that makes it possible without affecting the engine's mechanical stability.
Craig.