Kinda depends on what the intended use/class/cert you are aiming for.
For a simple NHRA 8.50 & slower legal full roll cage install 2x2x.125 square tube is kinda overkill going by what the rule book minimum is. The rule book basically says that any frame member (frame rail, or crossmember, or outrigger, or etc) that any required roll cage main structure (main hoop, windshield down bars, rear main hoop support bars) sits on/attaches to has a minim OD & wall thickness of either 2x2x.058 square tubing, or 1.625x.118 mild steel round tubing, or 1.625x.083 'moly round tubing.
In this example if no part of the full roll cage main structure sits on the actual main frame rails (they all sit on crossmembers an/or outriggers) then there is no minimum size for the main frame rails (and/or subframe connectors) and you can use tubing as small/thin as you want.
If you want to go faster than 8.50 then you need one of the SFI 25.4/25.5 spec chassis books. In this situation using 2x2x.125 square will either be overkill (main frame rail minimum, 2x2x.065 square, or 1.500x.065 'moly round, or 1.500x.108 m/s round), or not big enough (main hoop crossmember minimum, 2x3x.083 rectangle, or 1.625x.083 'moly round, or 1.625x.118 m/s round).
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