Had a good bit of fun with it, the main points id make are:
I'm being told in the top corner of the screen that I'm going at 60-120kmph, but it feels like I'm relatively slowly driving around spikes, I don't play many driving games, but I would say that generally you want to be doing something interesting such as, strategically picking when to boost, or breaking round turns both of which I wasn't doing. You also have space to boost without mentioning it anywhere? I'm guessing that you left it in from Chief's implementation?
The boxes are a fun idea, but I don't feel like they currently add too much to the gameplay, I put 2 on the truck, a wait a tenth of a second when the game starts to make sure they don't immediately fall of, then drive like they're not there. There's also no incentive to over achieve, no risk/reward or interesting decision making, which I feel the mechanic could easily add.
Overall I'd say that it feels like the game doesn't know what game its trying to be, I would focus on either speed, or carrying the boxes, or try to make the current intersection between the 2 feel less messy. All that said I do think there's potential with this idea.