I'm not an automobile historian, so I have no clue about this, but when Sonny get's killed at the toll booth the windshield of his car breaks like regular glass when it's shot. However, when Carlo is getting strangled and kicks the windshield it cracks but stays together like safety glass.

I was wondering at what point did car companies start using safety glass for windshields? It seems to me that this could be an anochronism in GF, but then again a bit of time had passed between the killing of Sonny and Carlo so I don't know.

I don't know if it's the same thing but is safety windows the same as bullet-proof windows?

If it is I know for a fact that Capone used them so that puts them around the 19teens 1920's

No, safety glass is basically two thin sheets of glass that have a layer of plastic between them that holds the glass into one clump when it breaks, instead of shattering the pieces all over the place.

The glass is also tempered (Sonny's wasn't, Carlo's was), which means that it breaks into thousands of tiny pieces rather than larger, sharp ones.

ah ok I see what you mean now.

According to a "history of auto glass" essay I just read History

So if the Godfather takes place between 1945 and 1955 the glass in Sonny's car should have been safety glass. Thanks for the info.

Cancer the only think is is that Sonny's car was a 1941. So I am assuming that the glass you are talking about was made "around the 1950's" so it is possible just based on the years of the car.

May have been 45-55 but Sonny's car was made in 41, the car Carlo was killed in may have been made in 48 (I don't know)

I could be wrong, but I would think that with FFC's strict attention to detail that he would even get the type of glass right. I mean, notice how the cars had gas rationing stickers and wooden bumpers.

I'm guessing at a much simpler explanation:
Sonny's car was a '41 Lincoln Continental, which is a genuine classic. The GF production crew probably rented it from one of the agencies that specializes in the movie industry. Before filming the scene, they removed the real safety glass and substituted plain glass (or sheets of sugar solution "glass," often used as a glass substitute in movies) so that the real (and hard-to-replace) safety glass could be restored after filming. But the car used in the Carlo scene was a '51 Plymouth Cambridge, a plain old junker that they probably picked up at a junkyard, so they didn't care about kicking out the real windshield. They probably dumped it after that scene.
You see something similar in "Goodfellas." Remember the scene where young Henry smashes car windows with a crowbar, pours gasoline through the smashed windows and sets the cars on fire? No doubt the production crew substituted fake glass so the windows would smash cleanly for that scene. Real safety glass wouldn't have smashed--it would have caved, and young Henry wouldn't have been able to easily pour gasoline into the cars.