Is it possible to force the transmission into overdrive?
 

Basically, I want to know if there is a way to FORCE the transmission into 5th gear with a switch that can be turned on or off to force or not force the car into overdrive. Apparently, there is a "limp home" mode that makes the transmission only work in 5th gear (in Drive) and I want to know if its possible to take advantage of this (by temporarily "tricking" the system into going into limp home mode), or otherwise find a way to force overdrive.

I have no idea if it will damage the transmission to take advantage of "limp home" mode, or otherwise force overdrive. I have an interest in saving fuel by forcing overdrive whenever I coast down long hills, hills which in Drive I can go 30 mph off the gas, where in Neutral I can go 40 mph. By forcing overdrive, there would be less engine braking, and I could possibly go 35-38 instead of 30. (I could just use the gas to go faster, but then I get 45-50 instant mpg instead of potentially 90-100) Then I can turn off "force overdrive" whenever I slow down or need power.

This is an incredibly bizarre and far-out topic, so I'm not expecting this to be possible, but if anybody has any ideas, I would be thrilled to hear them. This is something you see a lot more of on Dodge trucks with a cummins engine, much less on cars.