Hello Satnam,

As per your question , Yes you can pursue m.tech in mechanical engineering after b.tech in automobile engineering Just you need to qualify Gate entrance exam in mechanical engineering with valid score to pursue m.tech course.

For more information and update please follow the following link

https://engineering.careers360.com/colleges/list-of-me-mtech-in-mechanical-engineering-colleges-in-india

Hope this will help you a lot,

Thank you

Yes it is allowed but you have to give the Gate Exam for studying Mehanical Engg.

For more details please follow the given link 

https://www.inspirenignite.com/how-to-change-branch-in-m-tech/