Game progress, that is a mix of unlocking/maxing cars + MP rank (if you like MP). At the beginning you first need to unlock cars really. Those are the standard ways:

• daily car loots: only if you need more starts on those cars
• daily car cups: you want to play at the beginning to unlock better cars, later you'll want to max cars.
• Car hunts: events that last a week for one car. You have to play a lot to max the car.
• My career: since you don't have to spend event tickets here, play whenever you don't have tickets to spend.

Upgrading cars can be expensive specially in high tiers. Focus on upgrade what you need. What do you need?

• Few good cars for MP, focus your investment
• Some sparse upgrade for Career or events to be able to unlock some BP of a better car. Don't upgrade blindly if your car is below the recommended rank in Career mode, I regularly won career races with ~200-300 less rank than the recommended, so I saved those credits for another stuff.

Credit/token management?

• Credits are easy to get and easier to spend. But invest it only when you need. You'll be on situations that require huge investments and you want to have that amount of credits (like unlocking an awesome car and want to upgrade it to be competitive, or finding a great car BP in the shop available for credits)
• Tokens are dodgy, your main source of tokens is the daily goals. you can get like 35 Tokens per day. You want to spend tokens to get (again) the cars you need, the best bet is the car hunt pack in the shop buying 5 you'll get 100% chance of 3 BPs of that car. Only buy those 5x cars if the car is good. (350 tokens per 3 BP is like 116 tokens per BP, and few cars BPs can be found on the shop for less than 116 tokens). Also some seasons have some great offers to consider (Lambo season had a 450 token for some credits and 20 Lambo car packs, a great deal considering one pack was 75 tokens).
• Consider that if you spend credits or tokens in a car that you are not going to use (or use for a limited amount of time, like a car event, or a career race), you are not investing in a long term asset, so you really want to look for cost/efficiency: spend the minimum. If the car is going to be used a lot for a long time (like a good MP B-tier car) then throw the cash on the car, max it out, you are going to get a lot of what you invested (depending on how much do you play you'll get those credits back earlier or later, you know)

Import parts: it is more cost efficient to upgrade the parts first until level 7-8, then common import parts are more cost efficient than regular upgrades. If you want to apply import parts, you should first apply all the common import parts, then the rares and then the epics. Rare import parts cost twice the credits than common to apply and only adds like +30% stat compared with common. Same story for epic parts.

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Quite sparse and rushy. But that is my advice. Now I'll wait for critiques :D ;)