For a first game its a good effort, and you'll find that players are willing to stick with a developer that doesn't abandon them where updates are concerned. I would fix any game breaking bugs first, then worry about additional features etc.

Oh, I notice your facebook links from Steam are down.  Get them up and running!  Let others know how you are getting on with the game and keep the ball rolling.

Keep up the good work!

You game is not better then Extreme G Racing on Playstation2.

You only have tunnels in your map, and the colors are to dark.

Besides motor games are not so popular, hoover cars would be more popular ( cars with wheels are more difficult to program then no wheels or motors ).

If you have a hoover car, you should have a strafe boost, to go left or right to avoid things.

Also have items for pickup, or like FZero those turbo things, and some weapons, look to this FZero map its not so claustrofobic :

Why did you not make it for free ?, to see what feedback you get and as promotion.

Buy the XGIII on PS2 or look the movie as inspiration for your next title.

Not a real suprise that nobody bought it if i look to your movies.

Honestly I love this idea for a game and I honestly think if you had worked out some of the bugs/flaws before launch it could've achieved more commercial success. I've never been too picky about bugs in games but it sounds like a lot of the negative reviews could've been avoided merely by having some testers try and break things. Granted you are a one person team, it's difficult to balance all these aspects at once and honestly I think you did an amazing job for the most part. The game looks fun and it is rewarding to successfully dodge everything. Heres what I noticed:

1. Game breaking bugs (falling through the map)
2. Jumping mechanic
3. Music
4. Game not 100% at launch

Here's what I would personally do to solve each if you were to go back in time and relaunch this game:

1. Give a few free copies to people who you know would try and find bugs. Even just to get a small community of people going who are interested in the game and giving you constant feedback. It seems like you had a nice community for the most part, just didn't have many people trying to break the game.
2. Jumping is huge in racing games but jumping in this game reminds me of KOTOR (the singleplayer one) pod racing. That's not a good thing, jumping has to feel more aloof. I'm not the most well-versed in videogame creation so I understand there could be something that I'm missing here but I'm guessing that the jumping system you implemented was easier than creating a physics engine based jump. All in all jumping isn't satisfying and like another poster said, it's really a mechanic that people try to avoid.
3. Again echoing Rutin, music is key and especially the retrowave games like this. People LOVE the retrowave especially with racing, and the soundtrack really helps people live that 70's vice city feel. Having a cool soundtrack would've been a major delight.
4. I firmly believe if you had done testing prior and had done that huge reworking of the game before the official launch, you would've been much better off. People only refund the game if they feel that it's bad/not complete. An early access title could've helped reduce the amount of refunds. If you had planned the launch out more and timed it well, potentially getting marketing help before it was launched would've likely helped create a much more successful launch and therefore more incentive to keep working on the game.

These are just my opinions and what I noticed. Please take them as constructive criticism because I know all in all that creating, launching, developing and doing it all on your own is incredibly difficult and although you didn't get your smash hit success, I would not chalk this up as a loss. Honestly it's a cool game, just has a few kinks.

Also another thing I forgot to add is that those estimates are almost always wildly inaccurate. I personally wouldn't rely on these numbers most the time but you can get close to them if you create advertisements that deliver well. Although one thing to keep in mind is what media your consumers are using, and generally gamers aren't on Facebook or don't disclose it often there. I probably would've tried to do some more involved forms of reaching, like sponsoring some streams or trying to get an Indie booth at a convention. Twitter isn't a bad idea to reach gamers on either if you're thinking social media.