I’ve always been too embarrassed to ask, but…
Say you were driving forward and you slammed your car into reverse…
would the transmission really “fall out” of the car? Can this even happen or is is a euphemism?
(I have been wondering this for about 10 years…yes, I am automobile illiterate in many ways.)
No, the tranny external case will likely stay in place, however a goodly number of the internals will become profoundly angry with you, and the fellow at the transmission shop will scratch his head and ponder why you did such a thing.
Actually, he’ll be pondering what to do with the thousands of dollars you’ll be paying him to fix said transmission problem. Stripped gears and/or broken shafts would be likely. You’d be looking at a replacement transmission as yours would likely not be rebuildable.
I believe most autos won’t shift into reverse if you are travelling forward above a certain speed. The gearshift might move, but the tranny won’t perform the shift.
I got nothing to back this up, so several grains of salt are required.
Wow, another Mondeo transmission question I can attack right in a row! I can’t speak for all cars, but the Mondeo I’m currently using won’t shift into reverse until practically stopped. Yeah, the shifter will move, but it’ll kind of be in neutral until virtually stopped, at which point the transmission clicks into reverse.
On my first car – a 1984 Ford Escort – throwing it into reverse would stall the car, but nothing else seemed to happen as a result.
For other cars, yeah, keep looking for those grains of salt. 
If you have a manual transmission, it’s pretty much impossible to actually get it into reverse when moving forward. See here:
In my experience, a car with an automatic transmission will make a loud clunk noise and stall out, causing you to curse and sweat. The vehicle will then start and run as before, though. I’m not sure what is actually happening in the transmission when this happens, but I don’t believe it’s actually getting into reverse gear. I’m sure it causes a certain amount of damage.
On a modern electronic transmission reverse is not allowed above a couple of mph. In my experience this is done in one of two ways. Either a solonoid deploys blocking the shifter from moving into the reverse gate, or the electronic control unit will not activate the shift solonoids to give you reverse if the indicated road speed is above the max allowed.
In the old pure hydraulic days, I never saw a destroyed trans from shifting into reverse, but I can tell you that the wheels do come to a very quick stop, and it may stall the engine. This was with the engine at closed throttle, if you did it with the throttle wide open, things could get very ugly.
I used to do this with my dad’s '55 Chevy. In a tunnel. I wanted to hear it peel out. Did this quite a few times until the tranny went out. The tranny was pretty durable and I was quite stupid. Also, in the same tunnel I turned off the ignition for a few seconds and then turned it back on. Quite a bang in a tunnel!
(Yes the muffler went before the trans.)
(Sorry Dad)
I tried this with 3 rentals.
Not a single one did anything dramatic when shifted from D to R at 55 MPH.
2 just ignored me, and one stalled without any real drama. It restarted easily in neutral.
Reason number 135,754 not to buy a used rental car.
In the days before locking torque converters and the like, AJ Foyt tried this. His findings were that the car (a rental filled with auto company execs) filled with smoke and made unhappy noises.
I don’t recall the exact event chain, but they ended up having to travel several tens of miles back to their hotel in reverse!
My favorite experiment was with a rented Ford Escape.
I’m cruising towards a stop light at maybe 10 MPH, with the brakes on.
I knock the tranny into “N”.
I floor the gas, revving her to the governed maximum RPM.
I keep it floored while I shifted into R.
It wouldn’t engage at 10 MPH, so I kept the gas floored as I hit the brakes.
When she had slowed down to around 5 MPH ± 2, the tranny engaged, and the wheels basically locked up as it skidded to a halt, stalling the engine.
I shifted to P and restarted the engine.
5 seconds later, the light changed. Even though I had my foot off the gas when I shifted into D, there was enough energy stored in the tranny that it appeared to spin all 4 tires when the tranny engaged.
That’s reason 135,755.
Or ever drive with JW.