I see 165 lbs on the Model S, but it has a hatch. With all those batteries low, and no hatch, seems like a little more roof capacity would be... Possible.
I'm not sure what the batteries have to do with the weight rating of a roof rack but you also need to consider that the MS is made of aluminum which is stronger than the steel the M3 will probably be made of, so that would be a factor as well.
How is an aluminum frame stronger than a steel frame?
I was going by what I've read (and has been posted here, I think) that the aluminum Tesla uses in their frames is stronger than steel. Of course, going back and specifically searching now I'm seeing a lot of "...as strong as ..." and similar statements.
I'd like to take this opportunity to retract my previous statement on the M3 frame. Thanks for pointing it out andy.
Stronger than steel had been marketing fodder for a while now.
stronger then the other stuff at the same weight was put out and people forget the qualifier.
a 5 pound bracket can be stronger than a 5 pound steel bracket or carbon fiber or whatever it is your trying to sell.
7075 T651: yield strength 73,000 psi https://en.m.wikipediadotorg/wiki/7075_aluminium_alloy
1010 Cold Rolled Steel: yield strength 44,200 psi. http://www.matwebdotcom/search/datasheet.aspx?matguid=025d4a04c2c640c9b0...
Depending on alloy and temper the distribution of yield strength varies with some aluminum a being stronger than some steels, but mostly not. It depends on what you use.
If the geometry is identical, any steel will be three times stiffer than any aluminum. Stiffness is a bulk property largely independent of alloy. But the light weight of aluminum many times allows larger sections and stiffer geometry at the same weight.
Hope this helps. This data sheet shows a range of yield strengths: http://www.makeitfromdotcom/material-properties/7075-AlZn5.5MgCu-3.4365-...