The car required is the Koenigsegg (is that right?) Regera. It is no longer the most expensive car but is still the one you need. Since it costs an obscene amount of money that would require an insane amount of grinding and would leave you broke, there is a way to do it for free - and no, it isn’t an exploit, it’s a feature in the game:
1. Go to Street Racing HQ
2. Go to the Hyper Car section and select it
3. Scroll to the Koenigsegg Regera
4. Select it
5. Back out until you’re back at the HQ
6. You should have the Regera in front of you
7. Test drive it - there is a timer, so if you don’t get the 2km in the time limit, just try again until it pops.