NASCAR vehicles are aerodynamically designed to have more down force that a fully laiden 747 sitting on the tarmac, grams won't make a difference, if grams made a difference professional drivers would be constantly trying to lose weight. You don't hear about guys taking pre-race enemas to get rid of a few extra grams do you? You know why, because grams don't make a difference, neither with a couple dozen pounds.
I don't know how heavy a NASCAR car is compared to an IRL car, but in an IRL car I think 60-80 pounds could make quite a difference over the whole race. (A few years ago, one of Dale Earnhardt's cars was on display at the Seattle hydroplane race, and I was surprised how small the car looked; maybe the weight differential between Indy cars and NASCAR isn't as much as I would think.)
I've got to call you on that one, buddy. Try 2 1/5 tons of downforce. Last I checked a fully laden 747 weighed a few grams more than that. Now if you're talking about how much aerodynamic downforce a 747 has parked, it is close to zero unless the wind is blowing and the flaps are in negative position! But that's not what you were talking about.
Also, your analysis is flawed in another respect. Aero downforce doesn't create mass or inertia. It reduces top speed, but not acceleration.
Ryan Newman Credit: Autostock
"The car creating 5,000 pounds of downforce racing side-by-side is going to have more effect with one car next to the other.