I think I used a Viper. Haven't played for a while, but you just slow down around corners and don't go too fast, the Viper pretty much destroys every other car on the track.
You are putting race tires on, aren't you? I knew one person who thought they were disallowed by the rules.
I'm thinking of starting a brand-new GT game this December, as it will be 8 years exactly (December 2003) and we'll see if I can just defeat the HTS this time in one go since I'm asssuming I'm a better driver now.