The latest nuclear data are used to examine the sensitivity of the least squares adjustment of the ²³⁵U fission spectrum to the measured reaction rates, dosimetry cross sections, and prior spectrum covariance matrix. All of these parameters were found to be very important in the spectrum adjustment. The most significant deficiency in the nuclear data is the absence of a good prior covariance matrix. Covariance matrices generated from analytic models of the fission spectra have been used in the past. This analysis reveals some unusual features in the covariance matrix produced with this approach. Specific needs are identified for improved nuclear data to better determine the ²³⁵U spectrum. An improved ²³⁵U covariance matrix and adjusted spectrum are recommended for use in radiation transport sensitivity analyses.