Hendrick Motorsports driver William Byron has completed two days of testing the NASCAR Cup Series Next Gen car at Auto Club Speedway, becoming the fourth Cup Series driver to take the 2021 car out on the track.

The Next Gen car was on track for its fourth formal test in advance of its planned competitive debut next season — previously, Austin Dillon (Richmond), Joey Logano (Phoenix) and Erik Jones (Miami) took their turns piloting the car and providing crucial feedback to NASCAR officials.

But this time around, it was a new car altogether on the 2-mile oval. Dubbed “P3” internally — the third prototype in the development process — NASCAR officials have described this version as nearly 100 percent complete for debut in the 2021 Daytona 500.

“I’ve been happy with it, especially as we’ve been adjusting it more and tailoring it to this track – this is the biggest track it’s been on so far,” Byron said. “As soon as it started to go for us, I thought the steering felt better and the car felt more stable. It’s been fun the more laps I’ve been able to run.”

PHOTOS: Exclusive access into William Byron’s Next Gen test

Tuesday’s session ended early for Byron, though, after an incident early in a 25-lap tire run. It was something Byron acknowledged as a learning experience, comparing it to a similar situation that happened to him during Sunday’s race where he was able to avoid incident.

“We were probably six or seven laps into a 25-lap tire run. I had been a little free for a couple of corners, but nothing major,” Byron said. “That time, I just got loose and figured I’d be able to save it but wasn’t able to. It just came all the way around. I had a number of similar moments in the race Sunday and was able to drive out of it. That’s what caught me off guard the most. It’s part of testing though, learning where the line is with what the car can do.”

That’s the view John Probst, NASCAR Senior Vice President of Innovation and Racing Development, took as well.

“This is exactly why we test,” Probst said. “We were able to put almost 300 miles on the car the past two days and captured some valuable data. Because of the nature of a test, we have a lot more data available than during a normal race weekend, including the IDR (incident data recorder) and high-speed camera. We’ll take the car back to North Carolina and evaluate it. This gives us a good opportunity to make sure the car holds up as expected during an incident. We’ll review everything available to us and move forward.”

The Next Gen prototype’s new features include 18-inch, single center-locking lug wheels designed by German wheel manufacturer BBS.

RELATED: NASCAR moving to single lug-nut design for 2021

That was one of many adjustments that Byron had to familiarize himself with compared to his current No. 24 Hendrick Motorsports Chevrolet.

“It was tough to get a hold of at first, just how fast everything is,” Byron said. “The tire doesn’t have the same sidewall, so there is not the same amount of slip that you can hang the car out. You just have to get used to that timing and rhythm of when the car does step out, how quickly can you catch it when it slides the front tires, how quickly does it come back. All those things are a lot different from what we do now.”

A feature that Byron enjoyed, as well as one that he had raced in his earlier days of racing, was the car’s sequential shifting. A six-speed gearbox allows driver to tab the level forward and backward during upshifting and downshifting, compared to the traditional H-pattern, four-speed unit featured now.

“I think it will be better for us on restarts to focus on moves and not have to worry so much about shifting,” Byron noted. “And on road courses, you’re going to be able to be a lot more aggressive in your downshifts. That’s going to be a lot of fun.”

Byron’s crew chief, Chad Knaus, was also on hand for the two-day test. Knaus expressed his excitement for the significantly different setup and build of the car, specifically how the components are smaller and more compact.

The seven-time championship crew chief said he feels the teams’ abilities to tweak at the track, compared to the present, will be drastically different.

“I like the car, I think it’s really cool,” Knaus said. “It’s definitely taking a step in the right direction in terms of modern motorsports, so I think that’s great. We have a lot to learn as an industry about what this car is capable of. I really want to acknowledge RCR and NASCAR — they did a fantastic job in getting this car built to get it out here, I thought it was spectacular.”