When Dale Earnhardt Jr. climbed from his race car after June's race at Pocono Raceway, his knees wobbled and he struggled to catch his breath.

Denny Hamlin and Brian Vickers also were noticeably shaken by the heat inside their cars. More importantly, they knew temperatures were only going to get worse.

Heat and fumes always have been part of the stock car challenge, but some say it's worse with the new Car of Tomorrow. Teams work so hard to keep air from getting inside the car, drivers sometimes are left gasping for breath.

"Since we switched to the unleaded fuel (last year), the heat in the cars has gone up," Kevin Harvick said. "There have been a lot more fumes than what we had before."

NASCAR monitors five cars every weekend for heat and carbon monoxide fumes, competition director John Darby said. And so far, they haven't found anything out of the ordinary.

"You see teams that do a better job insulating the car and the temperatures are very moderate," he said. "You see the teams that don't and they're 25 degrees hotter. It's a matter of one team doing a better job than another."

According to Darby, the new car reaches about 115 degrees during a race.

"This car kind of builds up a lot more smoke than what we had before, so that is good to know that they are monitoring it," Harvick said. "It just seems like there is less air getting in to the cab for whatever reason. I don't know exactly why."

The new car is wider and taller than a traditional race car. To reduce the added drag, teams are funneling air away from the cockpit. Most drivers use a special unit that pumps cool air into their helmets, but it only goes so far at 115 degrees during a three-hour race.

"We've known this car is hotter," said defending series champion Jimmie Johnson. "We saw some of that last year, and as the summer months are coming along, it's really showing up."

Johnson said the first few laps are the toughest, especially since he wears flameproof underwear, a thick racing suit and full-face helmet that covers most of his body. Once he starts to sweat, however, the moisture keeps him cool. The final 20 minutes are tough, too, because the sweat turns to steam.

"Until you sweat enough and saturate the suit, then the air that does move through the car when the suit is saturated you have a little bit of an air conditioning effect that takes place," he said. "But the first 20, and then at the end of the race you're certainly tired, but until you sweat enough, which is kind of gross, but until you sweat enough, it's just not comfortable in the cars."

Last September at California Speedway, it was 114 degrees at race time. Inside the car, it was 145. California, Pocono, Dover International Speedway and Infineon Raceway in Sonoma, Calif., are considered some of the warmest tracks.

Jeff Gordon said there are things a driver can do to be better prepared for the heat. He watches what he eats, stays in better shape and makes sure he drinks plenty of fluids.

He also insists his Hendrick Motorsports team has added enough vents and fans inside the car to keep him comfortable.

"Now that we're running these cars every weekend, and the heat has really started to kick up; we've had to adjust," he said. "It's hot in there."

When Hamlin returns to Pocono in August, he said his team will be better prepared. He's running cool air into his helmet and his team has added a couple of vents.

"We kind of mix it up a little bit," he said. "Most everyone has fans. I've been running without them and finally we just got to a point where let's try to do everything we can to help the comfort level."