The speed limiter is a software limit set by the tune. If you set it low you can hit it like you stated 120 and the ecu will start cutting the power. set it to 299 and you will be able to reach that speed if your engine has the power to reach the 185 mph. easily done.
The speedo adjuster % will correct errors in your tire diameter. Say the difference between the 240/50/16 and the 240/55/16 (which is taller) All the ecu is doing is changing the SFPM formula By percentage of tire diameter variance from nominal programmed setting.