I just finished my 3rd year of Mechanical Engineering at Sydney Uni, and i must say that having a strong understanding of maths is vital for any engineering course.
If you are wanting to transfer to commerce you need strong marks to secure a place for the following year. Considering there are 16 out of 48 credit points of maths subjects in the first year, its best not to do Mechanical Engineering. The other 32 credit points are chemistry, statics/dynamics (physics), MATLAB/CAD subjects, and professional engineering crap. Nothing much has to do with cars till probably Thermodynamics in second year first semester.
Its best for you to try enroll into an economics course, where you could possibly do subjects which would count towards your commerce degree for the following year.