Currently, Speedtest.net does not have complete support for IPv6 users.

Our testing technology itself is protocol-agnostic, but there are a few remaining hurdles. Recording the test results from IPv6-only users isn't supported yet, so even though you'd see the result it wouldn't be saved to your results page. Additionally, we work with hundreds of sponsors around the world to provide over a thousand Speedtest.net locations, and and they have every hardware configuration under the sun, including many not yet compatible with IPv6. Supporting IPv6 would require routing IPv6-only users to only the IPv6-compatible servers.