Easy!
When you get the Ford GT Concept keep it and get it to the max hp that you can. And if you get high-gear kit it can reach up to 205mph. And if you use close-mission it can reach up to 135mph. I suggest you use high-gear in open tracks and close mission intracks that have tight turns.
More tricks, tips and cheats for this game are right here - Sega GT 2002 cheats
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