yes JEE mains is very useful for automobile engineering as most of the top colleges give  admissions on the basis of your rank in JEE mains. all the NITs and IIITs give admission on the basis of your rank in JEE mains. Degree through these colleges hold a great value and if you qualify JEE mains, you will sit in the JEE advance and if you secure a good rank in this you can get an IIT. All these government colleges are the best with the education systems and the exposure they provide with the best facilities to make your dream a reality. So if you want to do automobile engineering you should definetely give this exam. but remembers it requires a lot of effort and hard work to secure a good rank. Besides these there are some of the Private colleges that have there own exams like BITSAT for BITs which is also a very good college, then there are exams like VITEEE , SRMEE. But still i will say JEE mains is very necessary.

hey ,

EAMCET and JEE mains are exams which are written in order to get into engineering colleges for B.TECH courses. 

not all the colleges accept EAMCET or JEE mains. it  totally depends on which college you want to study in . but both the exams are helpful for getting into automobile engineering .

hope this helps .