Kung exhibited two bases for the subspace of bivariate polynomials spanned by the Tutte polynomials of matroids of size n and rank r, thereby determining its dimension, and asked what dimension subspace of bivariate polynomials is spanned by the Tutte polynomials of matroids of size n and rank r drawn from a fixed class C of matroids (such as cycle matroids of graphs or binary matroids). We give a sufficient condition for this space to be of the maximum possible dimension r(n-r) + 1, i.e. the same as that spanned by the Tutte polynomials of all matroids. In particular, we produce a basis composed of the Tutte polynomials of r(n-r) + 1 graphs, each of which is a series-parallel graph with a number of bridges and loops added. We also determine the dimension of the subspace of bivariate polynomials spanned by the Tutte polynomials of connected matroids of given size and rank. Again, an explicit basis is provided: it is composed only of Tutte polynomials of planar 2-connected loopless graphs. To obtain this, we make the intermediate step of providing an explicit basis of the subspace generated by the Tutte polynomials of all matroids of given size and rank that are connected after the removal of any loops. We additionally obtain, as a by-product, an explicit basis for the class of all loopless matroids of given size and rank.