Answer by maco1717 for How do I verify the speed of my NIC?

Also for future reference I've noticed that the speed field in ethtool gives the maximum speed supported by the NIC and mii-tool give the actual speed on which the NIC is running.

[ root @  ]# mii-tooleth0: negotiated 100baseTx-FD, link ok[ root @  ]# ethtool eth0Settings for eth0:        Supported ports: [ TP ]        Supported link modes:   10baseT/Half 10baseT/Full                                100baseT/Half 100baseT/Full                                1000baseT/Full        Supported pause frame use: No        Supports auto-negotiation: Yes        Advertised link modes:  10baseT/Half 10baseT/Full                                100baseT/Half 100baseT/Full                                1000baseT/Full        Advertised pause frame use: No        Advertised auto-negotiation: Yes        Speed: 1000Mb/s        Duplex: Full        Port: Twisted Pair        PHYAD: 2        Transceiver: internal        Auto-negotiation: on        MDI-X: off (auto)        Supports Wake-on: pumbg        Wake-on: g        Current message level: 0x00000007 (7)                               drv probe link        Link detected: yes

UPDATE: After a while a found out the mii-tool wasn't returning the right speed as it was outdated and deprecated and ethtool was returning the negotiated speed.