From what i recall from 1.6mm to 2.0mm there is an increase of 25% in weight for an increase of 30% in torsional rigidity.

Thank you both for that information, I really am bold over by the helpfulness of this forum. I hope I can contribute as much one day. Sorry I seem to have taken this thread off in another direction.

Not to make new post, I think is the good practice to mix square and round tubes. This could lower a wieght a bit and increase stiffnes. At least this is my goal to make when my build will start. Main structure 25x25x2mm and all braces and other with round tubes + some reinforcements.

My only consideration is if the roll axis and other performance bits change significantly if i will use MX5 front uprights, wishbones modded for that and rear sierra ones. I plan to use zetec (tuned to 180hp) with t9 and sierra diffs are way cheaper than miata. Coz i doubt that its possible to mix miata rear uprights and driveshafts with sierra diff.

cheers

at the end of the day, a few kilos here or there wont matter. for example as a little runt of 9 stone wet a 600cc bike is fast to me but slow to my 16 stone dad.(sorry dad)