After unlocking "Auto Modellista" you get to change certain car's engines. Since when you change an engine your not allowed to tune your car here is a tip to get maximum speed. As an example i took the Toyota Sports 800 and put in a 2JZGTTE. With just that alone the car tops out at 165 mph. Go to tune and then pick "easy tune" do maximum speed ++. this adds all performance parts to the car. After doing this i took the Sports 800 onto Osaka Highway and got it up to 210mph and it was still going!