Originally Posted by cpaharley2008
I often wondered if one uses their transmission to downshift to help slow down long hills, does the increase engine rpms indicate additional gas is also being used?
If you have a carburetor (meaning you drive a very old or custom vehicle), or if your fuel injection system does not shut off fuel flow during engine braking (a feature called Deceleration Fuel Cut-Off or DFCO), you will use more fuel due to the higher engine speed... but not a lot. Remember that the throttle is fully closed, so as little air (and therefore fuel) as possible is passing through the engine for each engine revolution.
My Toyota van uses zero fuel in this situation (it has DFCO). Although I never towed with my Ford Focus, I noticed that it kept injecting fuel while engine braking, instead of shutting it off entirely; this made me less enthusiastic about engine braking, but I still used it as appropriate to avoid overheating brakes on long descents. My current Mazda3 also keeps injecting fuel (which surprises me), and I do use engine braking in rare circumstances.
A diesel should inject no fuel when not applying any power, because it doesn't need to keep a constant air/fuel ratio. Unless there is some unfortunate minimum fuel delivery feature, a diesel will consume no fuel during engine braking when the driver's foot is entirely off of the accelerator, regardless of how high the engine speed goes.