Step are giving below which you have to follow in order to get 'Rental car'.
Go to nitro type and create a new account.
(I have written 'Create a new account' in bold because it is very important. Without creating a new account you will not be able to get that car.)
Do not join in any race.
Go to your profile page and click on 'Race With Friends'.
After clicking on that a race window will come with a single racer on it, that is you.
You do not have to invite any friend, Just click on red 'Start Race' button.
Wait for 10 seconds all the bots will join your race and you will be provided with a sentence of four five words.
Type that sentence and you are done. You have 'Rental Car' now. You can use it everywhere, you can join simple races of friend races, or Nitro Type Olympics, everywhere. :D
A short 3 minute video is given below. You can watch it for additional help.