Before you attempt this, check that the unit measurements in your game statistics page are in miles ("Imperial"). If not, change it by using options->game settings.
You'd need a fast car for this one. The Ascot Bailey that you get from Leo in Chapter 11 will come in handy.
After dropping Leo off at the staion, hit the bodyshop with it and upgrade/tune it twice. Drive to the Grand Upper Bridge (the twin bridges that's hard to miss on the map). Step on the gas and you should reach 125mph with some luck. Use the horn (press X) to move any vehicles blocking your path out of the way.