as mentioned by others, there is a feature on the DIC, which allows the driver to set a max speed for him/herself, which when exceeded, sets off a bell. this does not limit the speed, it only notifies you that you have exceeded your chosen speed. i forget if this is set in the "information" or "performance" screen, but if you did not set it, maybe someone else did, and you should go into the screen and see what is there. if your local speed laws permit, you could try taking your car to that speed, and see if you are able to exceed it. if you are not, then i have no idea what it could be. as mentioned by others, there is a 185 mph "governor" on the vehicle, which is set by manufacturer, as a function of tire limitations and attendant risks. this actually limits the attainable speed of your vehicle.
Note: it sounds as if you were driving a dealers vehicle for a test drive. given this, they may, for liability/insurance purposes have actually set the max speed to 85 mph, so you can ignore the above, and ask them what they did. my comments and those of all others are based on the typical vehicle, as you would take delivery, in the USA.