The input rate is a measure of the rate of traffic received from the far-end device attached to the serial interface in question. The output rate is a measure of the rate of traffic being sent to the far-end device attached to the serial interface in question.
The output of the T1 may not be the same as the input rate on the router uplink esp. if you have other connections on the router. Also the router uplink will likely have greater link speed than the T1 thus there may be some internal buffering in the router to account for this.
To determine how utilised the T1 is, the meaningful readings can be taken from the 'show interface serialx/y' command as follows:
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
MTU 1500 bytes, BW 1024 Kbit, DLY 20000 usec,
reliability 255/255, txload 20/255, rxload 61/255
Encapsulation FRAME-RELAY IETF, loopback not set
Keepalive set (10 sec)
LMI enq sent 771200, LMI stat recvd 770366, LMI upd recvd 0, DTE LMI up
LMI enq recvd 8, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
FR SVC disabled, LAPF state down
Broadcast queue 0/64, broadcasts sent/dropped 0/0, interface broadcasts 0
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters 12w5d
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 9
Queueing strategy: weighted fair
Output queue: 0/1000/64/9 (size/max total/threshold/drops)
Conversations 0/25/256 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 768 kilobits/sec
5 minute input rate 248000 bits/sec, 75 packets/sec
5 minute output rate 84000 bits/sec, 47 packets/sec
245353384 packets input, 3304209937 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
4 input errors, 0 CRC, 3 frame, 0 overrun, 0 ignored, 1 abort
130482007 packets output, 3417055172 bytes, 0 underruns
0 output errors, 0 collisions, 279 interface resets
0 output buffer failures, 0 output buffers swapped out
0 carrier transitions
DCD=up DSR=up DTR=up RTS=up CTS=up
The bits in the output that demonstrate how utilised the T1 is are the txload/rxload and/or the 5 minute input/output rates.