There is no guarantee that your modem will achieve this speed. In the real world any speed at or above 24,600 bits per second is a good connection. The v.90 and the other 56k protocols should adjust the connect speed to changing line conditions. Speed is not everything.
Some modems are very aggressive about setting an initial connect speed.
If this speed is too high for the line conditions it will result in errors and require the retransmission of data. Those retransmissions will eat into your throughput. A 33,600 bps that is stable will result in better throughput than a 48,000 bps connection that is flaky. An overaggressive initial connect speed can also result in retrains , a situation in which throughput stops altogether while the modems renegotiate the connect speed. There are a number of factors that can limit your connect speed.