There's a few, mostly closer to simulation than tux kart tho. A few that I'm fairly sure are under active development:

Stunt Rally.

VDrift.

Speed Dreams. Can't easily find the github for this one.

I suppose Rigs of Rods fits the bill, although I never played it.

The others I'm aware of (that still seem to get updates) are Stunt Rally and TORCS.

I put together this giant list of free Linux games a while back, which also catalogs if they are open-source and what license they are released under. From the Racing section, I would second @ryven@lemmy.dbzer0.com's suggestion for Sonic Robo Blast 2 Kart :)

Sonic Roboblast 2 Kart and its successor Dr. Robotnik's Ring Racers are open source, and pretty fun! The exact degree to which they're free-as-in-libre depends somewhat on Sega's policy of turning a blind eye to fan games, so although GPL-2.0 permits commercial use I wouldn't recommend testing it.

There is a lively modding scene (or there was for SRB2K last time I was playing it; I haven't played RR yet). Ring Racers has single-player content if that's what you're looking for, I hear it's quite challenging.

Media files are proprietary and it's under the zlib license if that disqualifies anything, but for a somewhat decent experience, there is H-Craft Championship. Not the greatest of games considering it's a game that originally released in 2007 as a commercial game available for Linux (don't know about other platforms outside of the 2015 open source(?) release) by some seemingly random independent studio (Irrgheist), but fun enough I guess.

The major ones are already listed, so I mention only a few lesser known ones:

• https://trigger-rally.sourceforge.io/
• https://sourceforge.net/projects/extremetuxracer/
• https://juzzlin.github.io/DustRacing2D/