Additive bonuses are generally bad in incremental games like this; there's a reason why Clicker Heroes is ALL multipliers. Only things that should be additive are limited stacks of similar amounts. I think the best example I could give with Idle Racing GO in mind is Crew bonuses: each Crew member bonus should be multiplicative, but their own bonuses should stack additively with levels. For example, a level 10 member with +50% Bonus Cash per level, and a level 20 member with +75% Bonus Cash per level, would give x6 (+500%) and an x16 (+1500%) multipliers, amounting to a total x96 multiplier. Without this, the +50% per level becomes irrelevant rather quickly. And of course, these multipliers can be balanced by a steeper difficulty curve, as long as we get multipliers so we actually see progress in the later game.

Another point I'd like to make about additive bonuses regards the +10% All Performance bonuses. They work pretty much the way I explained above, but the issue I can foresee is that once you reach the very late car parts, you'll have so much of that bonus that a single +10% will be worth practically nothing. I feel this bonus, especially being so small, should be multiplicative with itself, 1.1x 1.21x 1.33x 1.46x instead of 1.1x 1.2x 1.3x 1.4x. And if I might add, the +15% Bonus Cash should follow the same pattern as the All Performance bonuses, and maybe be reduced to 10% so as not to be too overpowered.

Automatic Ride, Hazardous Nitro and Extreme Nitro have cooldowns that are way too long for the benefit they have. Automatic Ride needs a lot of levels to become somewhat useful, and at that point it should be awesome not just somewhat useful. The other two skills I would merge together since it's already pretty stupid to not use them together, and even merged I would reduce their cooldown or increase their base effect so they're more useful early on. If you were to merge the nitro skills, then may I suggest a new skill involving "fast lane" chance of appearing and/or an increase in fast lane rewards?

The cost of firing a crew member should not rise with the number of crew members. IMHO it should never cost more than $1 worth of diamonds to fire a crew member, and I much prefer a system where the cost increases per use, i.e. the more you fire crew members, the more expensive it becomes. Examples: 10 diamonds for the first firing then +10 per firing, or 50 diamonds for the first then +2% (multiplicative) per firing, both capped at 200 diamonds.

Many Crew members are very lackluster but, since they're mainly related to the three subpar skills mentioned above, I think it's only a matter of balancing them along with whatever is done with the skills themselves.

Ivy is crazy overpowered. She's basically a feedback loop: the more you invest in her, the more you get currency to invest back in her. I mean, what other Crew member can literally double your Credit gains per hour with only 10 levels? I would seriously drop her Credit bonus down to +2% per level but make sure that this bonus is multiplicative with any other Credit bonus.

Diamond gains are a way too slow outside of first-time league victories and achievements. Once it gets to the point where you barely obtain any of the latter, it becomes hard to invest in gang morale or on car blueprints. I understand you want to sell Diamonds with MTX, but this is leaning towards the "cash grab" epithet. Here's a thought: how about we gain Diamonds every time we beat a League, but something like 25% of the amount after the first time which remains 100% obviously. (Edit: I think even 10% would be enough to make the game feel much less like a cash-grab.)

Finally, other currency gains are too flat across the Leagues, making it pointless to try and break a wall boss or even progress beyond the point where you no longer progress at max speed. For instance, Gang contribution points could increase by 50% (multiplicative) every league, so Bronze IV would give 1.5 per opponent, Bronze III would give 2.25, Bronze II 3.38, etc. Same thing for Credit, each League beyond Bronze V should give a 1.5x multiplier on top of the Tournament multiplier. To be honest, I was disappointed to see that the current "League" multiplier was additive to the "Tournament" multiplier which is the main reason why there's no point in pushing to the next League. I would merge them together for a straight .5x per Tournament multiplier, multiplicative with the .5x per League multiplier as described above. (Edit: actually, I'm thinking the multipliers for Leagues, including the Gang contribution one should be 2x not 1.5x, otherwise that probably still won't be enough to warrant taking the time to break a wall. Costs could be adjusted accordingly, just keep balance while making it interesting to break walls.)