6.4 Hemi Trucks

The new 6.4's have it in the trucks. this is from the Ram website
THE 6.4L HEMI® V8 ENGINE WITH FUEL SAVER TECHNOLOGY
The available 6.4L HEMI® V8 engine with Fuel Saver Technology delivers Best-in-Class gas horsepower+.
Fuel Saver Technology shuts down four cylinders when cruising or driving downhill under light loads to optimize efficiency without sacrificing power

It doesn't say anything about it in the 5.7's though