I'm going to give you my tips here in confidence, because I believe I've broken the game with how easy it is to win now:

Find a very low-PI car. In my experience, I've had amazing luck with the 51 Holden Ute, the 75 Ford Bronco, and the 70(?) Mazda Cosmo. Do an engine swap, AWD conversion, NOT aspiration. Upgrade the engine fully or as fully as possible. Upgrade to race brakes for sure and rally suspension if possible. Race transmission is a must as well. you will NOT be able to upgrade the tire compound, maybe up the front tire width if you're lucky. You can add/remove heavy wheels, body mods, and smaller engine upgrades if you're cutting it close with PI.

If done right, your car should now be the top of D or C class, while rocketing to 60 in under 4 seconds and topping out above 200mph. Especially with my D-class Ute, I'm playing on Unbeatable and I just can't lose. Fair warning though, the brakes are USELESS in all of these even with race brakes, and e-brake drifting is a MUST to turn. On the upside, I normally am making 8-12k xp from skills alone in races where 1st place nets you 5k.

If you use the Ute, enter in the "Utility/delivery" events, and you will drive exclusively against Ford Transit vans. I normally win by 15s in a three minute race, and that's with copious amounts of drifting.

Use this information wisely. And know that this works for Street Races but NOT midnight battles. I was suddenly in a race between my C600 Bronco and a S2 (803 I think) BMW M3. Still won, but had to drive like a cunt on the corners to block him.