Looking into building a frame for a hotrod I've seen different sizes used for different size vehicles such as 2" x 2" or 2" x 3" box tubing and circular steel tubing but I am unsure if a .188 wall is the norm. When building a frame is there a standard wall to use or is a .188 wall enough?
If building for a street vehicle, according to this forum post, they say that for 2x3, 2x4, and 2x5, 0.120" wall rectangular tubing is more than adequate for car frames. If you have to go a bit thicker to find what you want, even better.
If you are building for competition use, you'll have to look up what their regulations state. Every sanctioning body will have their own rules which must be met to run in their races.