The main reason was for dramatic effect (seeing Carlo's feet smashing through the windshield),and a little variety in death (Paulie had been shot in a car earlier, so this time FFC chose the garotte.) As a "practical" matter, if they'd stopped the car, Carlo might have smelled a rat and resisted. But I guess he wouldn't have expected to be killed while the car while the car was presumably moving to take him to the airport.
Also, what did they do with the body? By the last scene, Connie obviously knew that Carlo was dead. Wouldn't her first emotion be revenge by the Family? [/QUOTE]
This is an interesting question. I'm guessing that they dumped the car somewhere (it was an old clunker--a '49 or '50 Dodge, and this was 1955). But, did Michael order that the car be abandoned somewhere to be found--with Carlo's body in it, also to be found? Yes, if they wanted to advertise to their enemies that they found the rat in the family and dealt with him. No, if they wanted to avoid letting Connie know that her husband had been murdered, perhaps leading her to at least suspect that maybe, just maybe, he abandoned her. I'm thinking the former: Connie knew he was murdered, as in the last scene, and connected Carlo's death with the Great Massacre of 1955 ("Read the papers! Read the papers!"). Since she knew Michael was responsible for Carlo's death, she avenged herself on him, in her own way by becoming a floozie in GFII.