Today, Rocket League developer Psyonix announced a new pack of DLC cars based on Hot Wheels toys is coming later this month, on September 24, 2018. At the same time, a set of actual Hot Wheels toys based on Rocket League cars is also coming to stores sometime in the following October.
The DLC pack, the Hot Wheels Triple Threat, costs $4.99 and comes with three cars:

Three Premium Battle-Cars: Gazella GT, MR11, and Fast 4WD

Four unique decals for each of the premium Battle-Cars

Gorilla, Snake, and Spider Toppers

There is also some free content coming alongside the paid content:

New Hot Wheels Rivals Arena
Hot Wheels 50th Anniversary-themed customization items

On the actual, physical toys front, Hot Wheels is dropping a Rocket League 5-Pack in “participating stores” in mid-October. This set includes the Gazella GT, MR11, Fast 4WD, Bone Shaker, and Twin Mill III. These are diecast toys just like normal Hot Wheels, and certain, marked packs will include a DLC code for another in-game item, Hot Hot Wheels.

Finally, more on the Hot Wheels Triple Threat DLC Pack will be shown during a Hot Wheels-hosted livestream on the company’s official YouTube channel. In other news, today I learned that Hot Wheels does YouTube livestreams.

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