Found a glitch, you need to use Nitros for this. If you hold spacebar, spam N key, and up arrow and keep holding space, spamming N and up arrow, your car can easily go over 300 mph. My jetta goes over 370 MPH thanks to this.
Also, you don't need to buy nitrous, it is infinite.
Great game though. Looks like it needed some work before being released, still, very ahead of it's time.