If you are looking for a selection to the college, it will highly depend on if you have been called for an interview.
Now when it comes to institutes like IOCL, BPCL, NITIE, and many other PSUs, if you have an AIR of 1000 for mechanical engineering, your chances of bagging an interview will entirely boil down to the reservation system.
Lastly, if you are called for an interview, you are going to face a panel of experts who may or may not stick to your subject of preference. So, you should definitely brush up on all your syllabus. This includes general aptitude and mathematics.