The square root of PI
This is, ultimately, the most important factor when it comes to winning races. Yes, you can absolutely win races against cars with a bigger PI than you if you play perfectly, but that's less of a safe bet, frankly.
The bigger the PI, the faster your car will be able to go. When starting races you will actually be able to select from up to four different PI levels for your enemy, or difficulties. In the early game if you pull a good vehicle, you will likely be able to take down hard level races easily.
There is an extra-hard mode though, and that is a different beast entirely, best left until you've completed most of the content in the game.
Drift to boost
While in the races you want to build up your boost bar so you can go faster, of course, and you can do this by simply driving, but there are two ways to boost your, um, boost, even faster.
One is the perfect start. When you begin the race you will have to try and rev your engine to get the needle into the correct part of the gauge. If you do this successfully, you'll start the race with a small boost.
Perfect, and great drifts will also quickly build your boost gauge. Make sure to release your accelerator as your front wheels hit the brightest spot on the drift, and accelerate again when your back wheels hit the second bright spot.
Spending your boost wisely while driving is important. For one, only use the boost when coming out of a corner, there's no benefit to boosting mid-drift, or just before a drift.
Second, while it's great to boost away with a full bar, it might be wiser to spend the bar half at a time, allowing you to quickly build it up after drifts, and boosting more regularly.
If you get a perfect start you'll start with a boost, save it for after your first drift, and you'll likely have a full boost. Use this to boost to the next corner, and then use half boosts for the rest of the race. This should give you a good lead, as long as you're not taking on an insane difficulty level.
The upgrade path
We've already said that PI matters a lot, and that remains true throughout the game. But luckily, pulling cars isn't your only means of getting better PI. Yes, it's time to upgrade your vehicles.
Like in many other mobile games you'll find upgrade kits as you play through the game and clear events, and these upgrade kits can, of course, be used to upgrade your vehicles and raise their PI.
Once you've raised a vehicle a few levels, you'll need to undertake upgrade challenges to ascend any further. Make sure to take on all available challenges at your level, and get the most powerful cars you can to win races time and time again.