Manual cylinder deactivation?
 

Hypothetically, would it be possible to gain efficiency by disabling cylinders while cruising? I don’t see this working for several reasons, but could you hypothetically gain efficiency by installing a switch to shut off fuel to 2 of 4 cylinders while cruising under light load?

no

What he said.

You're getting short answers because this is been debated here repeatedly. If you use the search function at the top of this page, looking for cylinder deactivation, you are certain to find some of these discussions from the past. Lots of information there, and also a little heat in the discussion.

Mazda is doing it on the revamped 6 and CX-5 2.5L engines

VW / Audi is doing it on a 1.4 TFSI

Not manually though ;-)

And Chevrolet on the Silverado (previous generation and new 2019), and Dodge on the Challenger/Charger 5.7L, and RAM, and Ford on various Ecoboost engines, and...and.....

Cylinder deactivation is becoming fairly common these days, but it's a) computer-controlled, and b) uses mechanical trickery to do things like keep the valves closed on the deactivated cylinders. You can't replicate that at home without millions of dollars, a team of coders, and an up-to-date shop for fabricating.

But the OP is welcome to try!

Even better than cylinder deactivation is taller gearing.

In short Yes, and No.

Yes, because under low loads, you can increase the load by dividing it over less working cylinders. An increased load works like taller gears. The engine will be running more efficiently, because the throttle will be wider opened, and thus less vacuum is created in the remaining cylinders.

For this to succeed, you either need to find a real old car (preferably with an Inline configuration, as they're easier to balance, and usually have easier electronics to work with, like a carburetor instead of Fuel injection).
To stop the sparks from firing is easy.
To stop the fuel from flowing, not so much.

Or, get a modern I4 engine, but you'll need to do a lot of ECU bypassing and modifications.


and NO, because most modern cars have ECUs that will blink an engine light, and either shut down, or run into limp mode.
Not only will the engine be running rough, since it's not balanced anymore; but depending on the car, the injectors might be spraying fuel in the cylinders that are not detonating, causing fuel in the exhaust, damage to the exhaust system; and the exhaust O2 sensor might be leaning out the mixture, causing the working cylinders to run way too lean.

Dropping the RPM with a taller gearing while cruising sounds like a cost-effective approach. Not sure if the final results would be so similar to what the OP might be expecting, but still seems to be a safe bet.

Roughly speaking for every 12% in gearing increase, you'll get a 10% of MPG increase.
It's not linear, meaning you'll get better MPGs at lower speeds, and MPGs at highway speeds will be closer to identical; to worse (when the engine load is too high, and the engine is pinging or knocking).