If you’re looking to level up fast, you’ll need to generate heat.
The way that Rep works, it isn’t so much a currency as it is experience points to unlock better items for your car. The rep you pull in each night can be multiplied by how much heat you’ve accumulated with that evening’s activities.
There are a few different ways to earn heat, but largely you’re going to have to stir up some trouble. Evading capture in long pursuits, racing multiple times in a single evening and destroying pursuing cop cars will all jack up the heat, increasing your risk, but will also let you bag a significant amount of Rep much more quickly if you can survive the extra pressure from the authorities.
It’s important to judge this well because getting caught by the fuzz can be disastrous.