Many people are not even aware that Chitty Chitty Bang Bang was originally a book. Written by Ian Fleming (of James Bond fame) the book takes a much different journey. While on a drive in a newly renovated car, the Pott family happens upon gangsters who lead them on quite a chase around Britain and France. The family escapes on multiple occasions by following instructions from the car.
The film takes on an entirely different (and, it implies, imagined) adventure to a town in the fictional nation of Vulgaria where the people are living in fear of the horrifying Child Catcher. The car's occupants in the movie are Caractacus Potts, his children, and the father's sort-of girlfriend, Truly Scrumptious.
Essentially, you have two entirely different stories, the only common thread being the fantastic flying car. Even so, both stories are entertaining, albeit in different ways.
Also worth noting, there are three authorized sequels to book written in the last few years by Frank Cottrell Boyce.
Additionally, did you know the screenplay was written by Roald Dahl? I didn't until I did some fact checking for this post.