(GM sales: $193B) + (Ford sales:$178B) = $371B

US GDP = $11,750,000,000,000

So Ford and GM = 3.2%

I gues if you added in Chrysler and the American components of foreign firms manufacturing here that would bring it to 5%.

Plus you forgot AC Delco / Delphi, Philips Magnavox, Direct TV... to name a few under GM's vast umbrella.