I got gold on the Bus race and Kubelwagen race first try, but I sat down and did the Lotus one for over an hour one night and couldn't even get Silver. The Bus race you just need to keep your speed as best as you can while everyone else slows down, pass them. Same with the Kubelwagen race, but just barely touch the gas at top speed so you're not bouncing off the super slow rev limiter.
I remember after getting frustrated with the Lotus race, I looked around. Found a link that said to put TCS on 6-7, Active steering on, and around 5 ABS. I got Silver first try after, then Gold a couple more tries later. The AI cars tend to brake early, or places where you can go flat out if you can squeeze in between them. I always made sure I passed the last car through the first corner. Keep as much speed as possible in that little left curve going into Chicago, squeeze between them and you can gain another 2-3 places(you can pretty much go flat out here if nobody was in the way). Try to get past a couple on the inside of Chicago. Practice late braking for Hammerhead; easy to go too fast get disqualified, but perfect it and you can pass several cars here. If you're close enough(and careful enough), you can go flat out through Bently/Tirewall and get a place or 2 there. Bacharach is probably the easiest corner to move up places, they brake super early. Repeat on 2nd lap, just watch out for the idiots that want to bump/disqualify you. MOST IMPORTANTLY don't oversteer/spin tires coming out of hammer head and through the Follow Through, kills your speed/acceleration and any car close behind you will either hit you or pass. This challenge is mainly hard because of the bump/disqualify thing, I've been close with assists off but got ruined by bumping/getting bumped by an AI car.
Hope that helps someone.
