HOT WHEELS UNLEASHED

There are 5 "Secret" tier vehicles in the base game, and you will get all of them by 100%-ing the Hot Wheels City Rumble. I won't spoil what the cars are, but here are the requirements for each:

1. Beat all boss races. This will most likely be your first one.
2. Unlock all Secret spaces.
3. Beat all time trials.
4. Beat all races.
5. 100% completion. You have to win every race and get every Unleashed time on the time trials. If any space isn't done fully, it will be surrounded by red fire so it's easily found.

Some of the time trials towards the end can be quite testing, and you'll need top-tier cars to make the Unleashed times on some of them. Just take a break and come back to them when you have better cars if you need to.

Edit: Some top-tier cars include the 24 Ours, Koenigsegg Jesko, Night Shifter, Power Rocket, Legendary-tier Rocket Fire, the Batmobile, and Super Treasure Hunt variants of many of the base cars. Practice and know the tracks and what kind of car works best on them, i.e. tracks with long straights require cars that can hold more boost (like the STH Rodger Dodger), while tracks with lots of turns require cars with better acceleration and handling (like the 24 Ours).