Use the 75 x 75 x 600mm drive-in spike with a compatible 75 x 75mm wooden post and the 100 x 100 x 600mm version with a compatible 100 x 100mm post.
Our Samac drive-in fence spikes are sized to match the timber post section rather than using one socket size for both common wooden post dimensions.
The 75 x 75 x 600mm Drive In Fence Spike is specifically intended for a compatible 75 x 75mm wooden post. A 100 x 100mm post requires the larger support and should not be treated as compatible with the 75mm socket.
For a 100 x 100mm post, use the 100 x 100 x 600mm Drive In Fence Spike. It has the same 600mm spike length but a larger holder sized around the 100mm square timber section.
Both products have a brown powder-coated finish and are designed to be driven into suitable ground with the square post holder remaining above ground. Other fixing formats can be compared in our fence post spikes and supports range.