Sure you can. Using the bandwidth command you can change this.
Note: This does not change the amount of data that is allowed to go through the Tunnel interface. If you are trying to shape/police this you'll need to use other means. Bandwith settings on interfaces are more for dynamic routing protocols to decide which link should be placed into the forwarding table.
vxr3(config)#int tun0
vxr3(config-if)#bandwidth 10000
Tunnel0 is up, line protocol is up
Hardware is Tunnel
MTU 1514 bytes, BW 10000 Kbit, DLY 500000 usec,
Hope this helps,
Don
