He knows what he’s seeing, but sometimes Perry Fewell cannot believe his eyes.

The defensive coordinator of the Giants orchestrates what takes place with his unit and, taking full advantage of his embarrassment of riches, is the creator of the four defensive end look — dubbed NASCAR — that puts Justin Tuck, Osi Umenyiora, Jason Pierre-Paul and Mathias Kiwanuka — a linebacker who entered the NFL as an end — all on the field at the same time. Fewell has seen this grouping in action in the past and will unleash it Wednesday against the Cowboys, but there are still occasions when he is in awe of the damage it can inflict.

“When you talk about putting the four guys out there it’s hard to find out who’s the quickest and who’s the fastest,’’ Fewell said yesterday. “Some days I look and I say, ‘Boy, JPP looked great today, he looked awesome down there,’ and then Osi comes flying off the end and I’ll say, ‘Oh boy, that’s pretty fast.’ And then Tuck, he makes a move and I’m saying, ‘Wow, man, I’m scratching my head.’ And then Kiwi does something.’’

If Fewell is saying all this, just imagine what opposing offensive coordinators are uttering.

“‘Don’t put those guys on the field,’ I’m pretty sure that’s what those guys are saying,’’ Fewell said.

Tough luck. The gang’s all here for the Giants, four sets of eyes setting sights on Tony Romo, a familiar target they have pummeled before. Romo last season was sent to the turf repeatedly as the Cowboys were swept in painful fashion by the Giants. They got him three times in the first meeting and six times in the New Year’s Day division clincher.

“I think you absolutely have to game plan for those guys, and know exactly where they are on every snap,’’ Cowboys coach Jason Garrett said. “They have a number of different guys that can create problems for you, both in the run game and in pass protection. … We know who all those guys are, they have a lot of them.’’

When is “a lot’’ actually “too many?’’ There’s no doubt the secret is out on Pierre-Paul, who had 16 ¹/₂ sacks in his breakout 2011 season. Even more impressive, he has 21 career sacks despite starting only 12 games. When opponents devise a strategy for keeping the wolves at bay, the leader of the pack will likely be designated as JPP, who cannot be blocked by a single offensive lineman.

“I’m not worried about that,’’ said Pierre-Paul, who never seems worried about anything. “Probably when they scout you, they say, ‘He’s an All-Pro,’ but so are Tuck and Osi. They’re All-Pro’s too, so who cares?’’

If that is indeed the attitude all three take into the season — who cares who gets the sacks and the glory — then add selfless to a trio that can already be described as gifted and versatile. Umenyiora strikes like a cobra, sleek and swift. Pierre-Paul has the uncommon physical attributes, not as muscular as Tuck but bigger, longer, more naturally powerful. Tuck is a combination, built like a tank, smart and relentless, able to use a variety of moves and once he gets his mitts on a quarterback they rarely slip away. Kiwanuka, tall and sleek, is able to slither through traffic, use his long, long arms to secure his prey.

Last season, the foursome was rarely healthy at the same time. Tuck’s shoulder got a bit less angry in the playoffs and pass-rush dominance followed. Kiwanuka is coming off a strained groin but the four-headed monster is primed for a fast start.

“It was just tough to get us all three on the field healthy,’’ Tuck said. “I think those last games you saw us was us playing at the height of what it could have been the entire year. Hopefully we get there for 16-plus games this year and not have to have guys in and out, especially like how I was last year. A lot of pressure’s on us, but we like it that way. Hopefully we can set the tone early.’’

Fewell has come to expect as much.

“It’s a nice feeling,’’ he said. “When those four trot out there, I kind of smile.’’

paul.schwartz@nypost.com