You can find the Bandwidth utilization of an interface on a Cisco 3550 series Switch using the IF-MIB supported by it. Following are the details.
The MIB is IF-MIB ,
Object is ifSpeed and
the OID is 1.3.6.1.2.1.2.2.1.5.
I guess, it requires a minimum Cisco IOS version 12.1(12C)EA1