This is an amazing concept! The 2d sprite mixed with 3D gave me Crash Team Racing vibes, amazing for a pixel racer (although that was 3d characters). It's definitely in need of polish though, the game has low sense of speed. Not the actual speed itself, but that for example the tires could've been serparate sprites from the characters, and have a 2-frame or 3-frame animation where the tires' shine move a little (from, yep, CTR hahah). The ground being a flat texture and lack of boosting animation (some "wind break effect" maybe) also contributed to the feeling of slowness.
Aiming was surprisingly okay, after I learnt it's just about predicting where the enemies are going/their racing line, and for a 3 minute of action reminding me of Twisted Metal, the two weapons were good enough, but the big cannon also FEELS better to use - heavier sound, and really big area, in comparison to bullets, even though bullets take 3 round to tango while the big cannon only takes 2.
Think that's all I'll say about this one. Again, did have fun, and it's pretty cool how fast you whipped out something within 72 hours! Maybe I should participate in a jam myself, to actually make a real game. -shrug-