The use of large-mode-area tapered holey fibers with collapsed air holes for refractive index sensing is demonstrated. The collapsing of the holes is achieved by tapering the fibers with a "slow-and-hot" method. This non adiabatic process makes the core mode to couple to multiple modes of the solid taper waist. Owing to the beating between the modes the transmission spectra of the tapered holey fibers exhibit several interference peaks. They shift remarkable to longer wavelengths as the external index increases. The multiple peaks, combined with a fitting algorithm, may allow high-accuracy refractometric measurements which can be used for diverse applications.