We examine the Pioneer 10 data concerning the heliocentric radial variation of the interplanetary magnetic field. Incorporating the systematic increase in solar wind velocity observed during the mission leads to much improved agreement between the usual Parker Archimedean spiral and the data. The effects of correlations between short-period fluctuations of speed and azimuthal field are discussed and it is concluded that during the period under study they did not substantially affect the average magnetic field.