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!