Your wiki source sounds right. The high g like 4.5 to 5g is a combined result of vehicle weight and a downward force generated by air pushing the car down on the spoiler and other part, and a suck from fast air flowing under the car. In this case the track has to support the car with a force several times more than the car's weight, only at very high speed. In this case friction can exceed gravity. That is if you shell several hundred thousand Euros to get a car that does that. On a mass market for rest of us, we go for a car around tens of thousand Euros so the end result is close to what I referenced, maybe up to 0.7g. If you buy a 100,000 sports car and drive it fast (highway speed limit may to too slow unless you're on some of the roads with no limits in Germany), you could get beyond 1g braking. The simple answer, typical car + highway speed = 0.7g deceleration per some actual testing.