Assess yr current status before u take a leap. What was yr AIR in AIEEE ? Say, if it was 40,000 u missed IIT by a wide margin ( 35000 places). U wd need to bridge a wide gulf. Also, check out yr Performance Card of IIT when u receive the same. Suppose u get 70% of cut-off in each subject, u stand a chance.Be realistic. If the gap is wide, yr preprn needs to be that much more intensive. U shd have a bottomline: at least be thoroughly prepared for AIEEE & BITSAT. AIEEE wd be the fallback option.