NASCAR will debut a lower downforce package next month at Michigan International Speedway and also run it in July at Kentucky Speedway in anticipation of using it all next season.

The rules — which are not expected to be adapted for the rest of this season after Kentucky — are intended to take more downforce and sideforce away and lower corner speeds, allowing drivers to maneuver more around each other.

The changes feature:

  • A reduced spoiler that will be 2.5 inches high and 53 inches wide. The current spoiler is 3.5 inches high. The width is the same as the width of a spoiler for superspeedway events.
  • A tapered deck lid fin
  • Neutral rear toe/skew setting
  • 2-inch wide splitter with current center section length.

Gene Stefanyshyn, NASCAR senior vice president of innovation and racing development, told reporters Thursday that changes made before this season reduced the downforce on cars from about 2,700 pounds to 1,800 pounds. He estimated that teams have gained about 300 pounds back.

The changes for Michigan and Kentucky (and next year) are intended to take away about 500 additional pounds of downforce.  Stefanyshyn said a goal is to get the cars down to about 1,500 pounds of downforce, understanding that anything NASCAR takes away, teams strive to get some of it back.

"We've netted a better racing product, but we haven't netted some of the things that the drivers originally asked for, which was some more off-throttle time and some lower corner speeds to where they can actually race a little better,' said Scott Miller, NASCAR senior vice president of competition.

NASCAR tried the changes at a Goodyear tire test earlier this month with Martin Truex Jr., Aric Almirola, Austin Dillon and Kyle Larson. The smaller rear spoiler allowed teams to go slightly faster down the straightaways but the other changes caused them to lose about 10 mph in the corners, Stefanyshyn said.

Larson said the slower speeds in the corner is significant.

"There was a lot more off-throttle time with the lower downforce,' Larson said. "I think that's always a plus when you can lift sooner. With the current package … you're not ever all the way off the throttle. With the low-downforce, I was having to lift all the way off the throttle and if you want to call it coast into the corner both ends, which I thought was really good.'

Even if the changes work as well as hoped, Michigan and Kentucky likely will be the only races they'll be run this year. Teams could object to changes as the season enters the second half and closes on the Chase.

Michigan was chosen because it is an upcoming race. Kentucky has been repaved and teams will have a two-day test there. Each organization will be allowed to have one team there and NASCAR felt with the extra time there, it would provide a good chance to use the lower-downforce package.

-Dustin Long, NBC Sports-