So, in a nutshell, the system used in every game until now. You know, the one where you can upgrade a car in five minutes and take it for a spin.
Tweaking the XP values slightly isn’t going to fundamentally change the system. It’d still be an unbearable grind. It’d still be the same system, it’d still make no sense, it’d still be unfun and unfair. As far as I’m concerned, the only balance is either get rid of it entirely or this.