The only reason the chassis swaps from GT5 may not work are for two main reasons:
1. The tyre scaling is no longer the basically same across every car, where it pretty much was in 5. (You now need to adjust for each car accordingly)
2. PD have made some of the cars using incorrectly sized chassis, compared with the real-world models. (280z & 330i spring to mind, at least in GT5 they were completely wrong, with the 280z being on the same size chassis as the 240z, despite the real models having over 200mm differences in wheelbase measurements)
So far, these are the only two reasons any of my swaps have not worked properly when recreating things I've made in GT5.
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