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To be honest, an almost stock 135i shouldn't be hitting 184mph in the first place, that seems a bigger bug in this game!
It seems to me that the bug could be the "flat floor", as far as the physics are concerned, is installed by default. Then when you install it, it reverts to "stock" instead. I.e., the logic / data switching is bkcawdras. I've not tested any cars myself, though.