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Belinda the Beetle by Rev. W. Audrey, the creator of The Railway Series?
Sorry, I don't think that's it. I'm pretty sure the main character was a Mini. Also, I don't remember anything about a jewel robbery. I think that the illustrations inside the book were in colour.
Thanks for the information though. I didn't know that Rev. Awdry had written books other than The Railway Series.
An illustrated children's book that was read to me when I was small some time during the 1970s in the UK.
The main character is an Austin Mini motor car that shares a garage with a vintage Bentley. The Bentley is boastful and constantly tells stories of his past as a racing car and his winning a major race (either heavily implied or outright stated to be Le Mans - Bentleys did indeed win Le Mans on numerous occasions throughout the 1920s).
During the story the Mini takes the family who own him on a day trip to the beach, where they are cut off by the tide. The Mini somehow manages to rescue them. When he returns to the garage he modestly doesn't tell the Bentley about rescuing the family.
I remember the cars being illustrated in a fairly realistic way (possibly with pupils in headlamps for eyes).
The Bentley may have been something along the lines of a 41/2 litre or Speed Six.
The Mini may have been one of the sporty Austin/Morris Cooper models.
As you can probably tell, I was car mad when I was a kid (and still am a little).
Edited by Aspectifer