Take the car to any body shop and upgrade the car's engine completely. With your new supercharged ride start any mission and head to the Grand Upper Bridge in Uptown. You won't be taking the bridge, rather the highway that curves off to the right of it leading to Southport.
Drive down that highway at top speed. Stick to the center of the road and honk your horn to make the cars move out of your way. Your velocity counter should reach 2000 just before you round the bend in Southport. You need to slow down below 75 mph and let the velocity score register before it will count. Remember any crashing will probably void the score and you will have to repeat the process.