This only work in Keys To The City, which is available for download from the Xbox Live Marketplace. Go into Keys To The City and put on all skills at full, then go to the highway and spawn the Agency supercar. Get in it and start driving. The top speed of the Agency Supercar at 4 stars is about 225 mph. Press Select to open up the "Keys To The City" menu while you are driving and scroll to "Change Driving Skill". Press Y to put it back down to 0 stars while you are still driving so the car downgrades to its original form. The speed of the Agency Supercar will increase to over 375 mph, even 400 mph if you do not hit anything. However, if you stop or increase your driving skill, the speed will return to normal. The speed of the Agency Supercar at 0 stars is 156 mph, but, with this trick the top speed can reach over 400 mph at 0 stars.