Tailenders

They pratically are the same movie! The animation style is very similiar, so's the story, and they're both about racing. Some might call the other a rip-off, I was just glad to find anything similiar at all. 

Car racing, a headstrong lead male, a mysterious lead girl. 

Both are fast paced high stylized and polished science fiction anime about fast cars, and both would make for a fantastic double feature. 

Simply said, if you want more of Redline, just watch Tailenders. Both are about crazy over the top races with similar style of animation that uses almost the same artstyle and usage of color palletes. 

Basically Tailenders is just Redline in 27 minutes. -Story takes place in an alien planet where a race about to begin which can be regarded as a quite unique plot. -Both of the movie is based on a crazy bizarre fast-paced intergalactic racing tournament with lots of adrenaline rush moments. -Both of the movies have almost the same animation style and they just crashes the colour palate all over the place in both cases and that ends up being the coolest thing you'd ever see. -The protagonists share almost similar characteristics.  

Needs no explanation. Both center around high speed galactic racing, featuring hot shot (literally) protagonists who will do anything to win and are motivated by beating some previously unbeatable figure. Both protagonists are accompanied by a beautiful, wise, and carefree female partner who plays a substantial role in their success. Both also feature a superb blend of 2D and CG, coming together to create an amazing visual experience, and cap it all off with an epic final sequence. If you really want to be a reductionist, calling it "Redline in 27 minutes" wouldn't be too far off from the truth.  

Short movies with similar aesthetics and designs, set in worlds were nothing really needs to make sense as long as it is as over the top as possible.