After earning 135 stars in Career Mode, a new game mode called Mastery will be unlocked. Mastery is a new way to make your car better, and make you better with that car. All you got to do is have the car. Seems simple! The higher you rank your car, the better the rewards. The more races you do, the more licenses you can get. Higher up cars give more licenses. For the top car that you used to be able to purchase, the 2015 GTA Spano gives you 100 licenses. 100!!! The best currently is 65 by the Devel Sixteen Prototype! But if you get the Veneno, you'll get a maximum of only 18 licenses. The licenses are spread out, like the Volkswagen W12. The W12 gives you a total of 33 licenses. On Challenge 3, it gives you one of them. Challenge 6 gives you another one. Challenge 10, however, gives you 7. Challenge 12 gives you 4, and the last challenge gives you the rest of the remaining (20). So, you don't have to immediately spend big bucks on expensive cars. Certain rewards include decal packs, more storage spots, and even cars. Here's the rewards and the amount of licenses you need to get the reward.
Shocking Speed Decal Pack - 5 licenses
Inner Strength Decal Pack - 20 licenses
Speed Lord Decal Pack - 50 licenses
Prowling Panther Decal Pack - 125 licenses
Unlock +10 Storage - 250 licenses
Camaro Z/28 - 300 licenses
Wild Stallion Decal Pack - 400 licenses
Unlock +10 Storage - 500 licenses
After Dark Decal Pack - 550 licenses
Datsun 280Z - 650 licenses
Extra Decal Pack - 750 licenses
Unlock +10 Storage - 750 licenses
Dragonflame Decal Pack - 900 licenses
Unlock +10 Storage - 1,000 licenses
Shelby Cobra 427 - 1,050 licenses
Alpha Wolf Decal Pack - 1,200 licenses
Unlock +10 Storage - 1,250 licenses
Molten Core Decal Pack - 1,400 licenses
Unlock +10 Storage - 1,500 licenses
Cadillac 16 Concept - 1,600 licenses
Darkest Knight Decal Pack - 1,750 licenses
Unlock +10 Storage - 1,750 licenses
Soaring Eagle Decal Pack - 2,000 licenses
Unlock +10 Storage - 2,000 licenses
Unlock +10 Storage - 2,250 licenses
Unlock +10 Storage - 2,500 licenses
Unlock +10 Storage - 2,750 licenses
Unlock +10 Storage - 3,000 licenses
Unlock +10 Storage - 3,250 licenses
Unlock +10 Storage - 3,500 licenses
Unlock +10 Storage - 3,750 licenses
Unlock +10 Storage - 4,000 licenses
Unlock +10 Storage - 4,250 licenses
Unlock +10 Storage - 4,500 licenses
Unlock +10 Storage - 4,750 licenses
Unlock +10 Storage - 5,000 licenses
Unlock +10 Storage - 5,250 licenses
Unlock +10 Storage - 5,500 licenses
Unlock +10 Storage - 5,750 licenses
Now, since there is so many cars, Gameloft has decided to fill the rest of the licenses with unlockable storage. It's kinda stupid, because it's just 10 slots. And it's 10 slots every 250 licenses. You will have to claim the rewards before they are deposited in your inventory.
You can earn cars by collecting licenses! The Camaro Z/28 is rewarded at 300 licenses, the Datsun 280Z is rewarded at 650 licenses, the Shelby Cobra 427 at 1,050 licenses, and the Cadillac 16 Concept at 1,600 licenses. Now you think that the higher the licenses needed, the better the car will be and the better the rewards. Well, that is not the case. In fact, the Camaro Z/28 gives you ZERO licenses, but at least it's nice enough to give you three extra tanks, which aren't very easy to get. It also gives you four tuning kits and six nitro starters, which is okay... but you should get licenses so it's a little easier for you to get the next reward. It took me two years to get the Z/28, so I imagine that I'll be dead before I get the Cadillac 16 Concept.
Mastery racing is a great source of farming races. The P1 has an good farming race, with The Great Wall, in Unplugged mode, earning an average of 6,000-6,100 credits earned each race, in about a minute, tops. Before the P1 was MAX/PRO, the GTA Spano was my go-to race for farming. Challenge 14 took place in Dubai Reverse, Classic mode, averaging around 5,550 credits in an astonishing 55 to 60 seconds. If you take the shortcut, that no other AI's take (if they do, they take the really long and curvy route before the bridge with the arcs above it), and wreck, you'll undoubtedly still be able to win the race. Yes, you'll lose 35 credits, but does 35 credits matter when you are looking to buy a 4,000,000-credit car? I mean, honestly yes.
While racing, be mindful of traffic. They are in every race and are there to get you and get you good. Three game modes are implemented in Mastery, which include Classic, Flawless, and Unplugged. Flawless is the hardest, as you have to shake off wrecks, and get first place at the same time. It's best to learn where traffic is at the location of the challenge, and try to avoid them. Unplugged is a race where all the nitro on the track is turned off, and it's up to you to say how much you want it. And Classic, well I'll let that go because you know what it is.
Along with the licenses and boosters, you'll be granted a one-time credit or token reward for completing the challenge. Depending on the car, you can even get Pro Boxes! The LaFerrari is the cheapest car to purchase and upgrade that gives you the rarest box period. One Champion Kit Box. It grants 30 cards, with 10 of them being Rare or Legendary. This box costs 950 tokens itself, but it's nice the LaFerrari gives you it at Challenge 10. Token rewards are usually saved for the higher-up cars, like the McLaren MP4-31, where two token rewards are available, distributing an amazing 725 tokens throughout it's Mastery.
Also to mention, in order to unlock the next challenge, you will need to complete the race before it, or meet or exceed the recommended rank.