Barely is the line between Nextel Cup racing’s very best teams and the rest of the pack as easy to draw as it is at the midpoint of the 2004 season.

Through 18 races, three teams have combined for 10 wins. They are also the top three in top-five and top-10 finishes and, not surprisingly, are the top three in the standings.

Jeff Gordon has the most victories, with four, and the most poles, with five so far this year, but both Jimmie Johnson and Dale Earnhardt Jr. have more top-five finishes.

Johnson and his Chevrolet team, in fact, have been incredibly consistent. If not for a wreck at Dover, Johnson would have almost certainly finished the season’s first half with a remarkable string of 11 top-five finishes.

So Johnson gets the A-plus in our annual midterm report card for NASCAR’s top series, while Gordon and Earnhardt Jr. have to settle for your run-of-the-mill A’s.

Earnhardt Jr. has nine top fives, but his team’s apparent difficulty in solving the riddle presented by 1.5-mile flat ovals is troubling as the season begins to build toward the 10-race Chase for the Championship at its conclusion.

Gordon has won the past four poles and pulled out a fourth-place finish Sunday at Chicagoland (Ill.) Speedway to keep his momentum going. Despite having won four times, Gordon has three did-not-finishes and has been in the top five “only” seven times. Johnson, by comparison, has 13 top-five finishes overall.

Behind that trio, here are the rest of the first-half grades:

A- Tony Stewart and Matt Kenseth: Kenseth bolted out of the gate while Stewart took a while to heat up. Stewart’s win at Chicagoland was a boost his team needed, but he’s fourth in points because he’s been running at the end of all 18 races. Kenseth won twice early and his No. 17 Ford team has sputtered a bit in recent weeks, but he’s where he needs to be for the next eight races until the title chase field is winnowed.

B+ Elliott Sadler, Kurt Busch, Ryan Newman, Rusty Wallace and Mark Martin: Sadler is in the best shape, championship-wise, among this group, which is bunched because each has won a race and that ought to be more important than it is under any points structure NASCAR seems willing to adopt.

Sadler probably needs about three more top-10 finishes to go with the seven he already has to secure his spot in the title chase. Newman has eight top-10 finishes, but also has four DNFs. Busch also is in the top 10 so far, but Martin and Wallace need to get hot and stay hot for the next eight races to make the cut.

B Bobby Labonte and Kevin Harvick: Neither has won a race, but they’re both right now in the top 10 in points and that’s better than a kick in the head. Labonte is sixth and Harvick is eighth, and a win for each of them would certainly help create the right kind of momentum.

B- Kasey Kahne, Jeremy Mayfield, Dale Jarrett, Jamie McMurray and

Casey Mears and Michael Waltrip: No room for error in the next eight races for any of these drivers. Kahne has flashed great potential, and it seems like a matter of time before he’ll continue the streak of seasons in which a rookie has won at least one race.

Mayfield has knocked on the door, too, and right now is 11th in points.

McMurray has nine top-10 finishes, but five DNFs. Jarrett and Waltrip have shown signs of improving in recent weeks, and Mears has show similar promise from the middle of last year to now.

C Terry Labonte, Brian Vickers, Sterling Marlin, Greg Biffle, Robby Gordon, Jeff Burton, Ricky Rudd, Brendan Gaughan, Joe Nemechek, Ward Burton, Scott Wimmer, Scott Riggs,

Dave Blaney and Ken Schrader: If this were the Tour de France, these guys would be the peleton.

D Ricky Craven, Kyle Petty, Jeff Green and Jimmy Spencer: Struggling to keep up with the pack, let alone to contend.

Other grades

• Incomplete — John Andretti, Bill Elliott, P.J. Jones, Bobby Hamilton Jr., Derrike Cope and Kevin Lepage: Andretti shows promise in the third Dale Earnhardt Inc. car. Elliott hasn’t raced enough to give a good read on performance, but he’s loving life in semi-retirement. The other guys at least try to compete when they show up.
• Present — Morgan Shepherd, Kirk Shelmerdine, Hermie Sadler, Chad Blount, Larry Gunselman and Andy Hillenburg: This group shows up and tries to bottom-feed when NASCAR needs field-fillers.