Redshift drift in varying speed of light cosmology

We derive a redshift drift formula within the framework of varying speed of light (VSL) theory using the specific ansatz for the variability of c(t)=c0an(t). We show that negative values of the parameter n, which correspond to diminishing value of the speed of light during the evolution of the universe, effectively rescale dust matter to become little negative pressure matter, and the cosmological constant to became phantom. Positive values of n (growing c(t)) make VSL model to become more like Cold Dark Matter (CDM) model. Observationally, there is a distinction between the VSL model and the ΛCDM model for the admissible values of the parameter n∼−10−5, though it will be rather difficult to detect by planned extremely large telescopes (EELT, TMT, GMT) within their accuracy.