Determine ethernet speed in RHEL 6

How to check whether eth0 has connection speed 10Mbit, 100Mbit or 1Gbit? I tried ethtool, but it says No data available. I also tried dmesg | grep -i duplex and it's empty.

[root@dioptase ~]# lspci
00:0a.0 Ethernet controller: Digital Equipment Corporation DECchip 21140 [FasterNet] (rev 20)

[root@dioptase ~]# ethtool eth0
Settings for eth0:
No data available

[root@dioptase ~]# ethtool -i eth0
driver: tulip
version: 1.1.15
firmware-version:
bus-info: 0000:00:0a.0

[root@dioptase ~]# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:15:5D:6F:1E:09
          inet addr:192.168.140.106  Bcast:192.168.140.255  Mask:255.255.255.0
          inet6 addr: 2a00:1120:0:1002:215:5dff:fe6f:1e09/64 Scope:Global
          inet6 addr: fe80::215:5dff:fe6f:1e09/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:95671897 errors:6 dropped:0 overruns:0 frame:6
          TX packets:16524440 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:23933711964 (22.2 GiB)  TX bytes:19761966217 (18.4 GiB)
          Interrupt:9 Base address:0xe000