I don't remember this well enough to rate it.

Immortal Grand Prix, on TV Asahi (Japan) / Cartoon Network (USA)
ANN; BCDB; IMDb; Toonami Wiki; TV.com; TV Tropes; Wikipedia; YouTube

This was Toonami's fourth "Total Immersion Event," and the first not to involve the programming block's CGI host, TOM, the captain of the spaceship Absolution, or the ship's AI, SARA.

Immortal Grand Prix (or IGPX for short) was originally an anime microseries in 2003, consisting of five 5-minute episodes. I should say that IGPX is pretty revolutionary in that it is a coproduction between Cartoon Network and Japanese animation studio Production I.G. Such American/Japanese coproductions are not without precedent, but I think this is the first time an American and a Japanese company have worked so closely together on an anime project, from the very beginning.

The series is set in 2035, and involves competing teams of battling mechas, three on each team. The main character is 14-year-old Takeshi Noa, on Team Suzaku. They're fighting against Team Sledge Mamma. I'm afraid that's all I can say, because I didn't get to see much of the microseries, nor do I remember anything specific about it. So I should try to watch the whole thing someday.

In 2005, there was a full-length series of the same name, which was set in 2049, and was different in several ways from the microseries.

micro-series index

Total Immersion Events
The Intruder * Lockdown * Trapped in Hyperspace * IGPX * The Intruder II * The Intruder III * Countdown * The Forge

Note: since I lost access to Cartoon Network in 2011, I haven't seen any of the TIEs that aired after Toonami's return in 2012.