This makes me wonder, why don't you ever see 6 wheeled cars on the road? Seems like there'd be some cool aftermaret for that and with 6 wheels you could get going that much quicker off the line.
Cars don't have lots of throw away room. The extra axle would impede on doors or seats or a trunk or whatever. That's why trucks are better

Bah, the 300, charger, or challenger have plenty of room in the back behind the rear wheels for another set.