A fastening system (1) for fastening an auxiliary wheel (2) to a main wheel (3) of an agricultural vehicle comprises an annular spacer (18) interposed between a rim (4) of the main wheel (3) and a further rim (17) of the auxiliary wheel (2), and a plurality of tensioning devices (33) acting between the auxiliary wheel (2) and the main wheel (3), for holding the auxiliary wheel (2) pressed against the main wheel (3). In order to reduce the stress to which a disc (5) of the main wheel (3) is subjected, a fastening element (24), or auxiliary disc, is provided comprising a first fastening portion (25), which can be detachably fixed to a hub (6) of the vehicle and a second fastening portion (28), which is axially spaced from the first fastening portion (25), wherein each of the tensioning devices (33) comprises a first end element (34) that can be detachably fastened to the second fastening portion (28) and a second end element (36) that can be detachably fastened to the annular spacer (18).