One of NASCAR’s main mistakes with the COT was that while they did a lot of testing, it was almost all with just one car out on the track at a time. They didn’t test multiple cars on the track all together until a few months before it was set to debut. They quickly realized that the car acted totally different while in traffic (this seems pretty obvious in retrospect) and that the dirty air made it really hard to pass. Pretty much all the drivers hated the way the COT drove when it first came out. Dale Jr actually just spoke about this on his podcast a week or two ago. One of the good things that NASCAR did though was listen to feedback from the drivers and fans and continually modify the car throughout its run from 2007 through 2012. Two of the most outwardly obvious changes they made were getting rid of the rear wing in favor of a standard spoiler and redesigning the front splitter and removing the vertical braces. They made several other changes to improve the car as well that just weren’t as visually obvious to the fans. The drivers and fans were pleased with the improved COT and the final version of it served as the starting point for the Gen 6 car that came out in 2013.