To find the correct cylinder numbering for *your specific* Mustang's 3.8L engine, you need to:
* Consult your owner's manual: This is the most reliable source. The manual will have a diagram showing the cylinder arrangement and their numbers.
* Look at the engine itself: The numbers may be cast directly onto the engine block or cylinder heads.
* Use a reliable online resource: Some websites specializing in Mustang repair or parts may have diagrams for the specific year of your Mustang's 3.8L engine. Make sure the year and engine code match.
Without knowing the year of your Mustang, providing a definitive answer is impossible.