In a non automatic setting it is not uncommon to just skip gears.
That said, in a 0 to 100km speed dash that is likely not normal.
The way i see it, you have 2 optipns.
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build from source, swap back to phys x. And live happy.
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build from source, audit and modify chaos to make it work. Realize it will never work and will always perform way worse than what Physx did. Go back to point 1 after having wasted months of dev, or release a game that wont work right on 90% of computers.
Someone may actually give you some tip to get chaos working - but the gear skip is probably the last of the problems to worry about when your vehicle feels like a doughtnut bouncing around the screen and when you get 10fps with 40 vehicles in a race…