The effect of a drift electric field on the spin transport in a heavily doped Si channel is investigated using nonlocal devices. A simple method to accurately quantify spin drift is established, using the ratio of the nonlocal spin-valve signal for two measurement configurations that have a different electric field pattern in the channel. The spin-transport length is obtained as a function of the electric field, and it is found that in heavily doped Si, drift electric fields of ±400 V/cm modify the spin-transport length (either up or down) by about a factor of two, relative to the length scale for purely diffusive transport. Although the trends are in agreement with the theory for spin drift, a quantitative comparison reveals that the theory significantly overestimates the effect of spin drift (by a factor of three). This highlights that an accurate experimental quantification of spin drift, as provided here, is crucial for a correct assessment of the role and importance of spin drift in practical spin-transport devices.