The ALTGX MegaWizard® Plug-in Manager allows you to enter values about 64Mbps greater or lesser than the maximum and minimum data rate limits supported by the Stratix IV GX device for basic functional mode. You can enter these values in the field What is the effective data rate? in the General screen.
For example: When you configure the Stratix® IV GX transceiver in basic single-width configuration with a channel width of 20, the maximum supported data rate for this configuration is 3750 Mbps. The ALTGX MegaWizard® Plug-in Manager allows you to enter values up to 3782 Mbps in the field What is the effective data rate? that is outside the supported maximum data rate limit.
Altera recommends you to refer to the Basic Functional Mode section in the Stratix IV Transceiver Architecture chapter in volume 2 of the Stratix IV Device Handbook to understand the minimum and maximum data rate limits in basic single-width and double-width configurations.