Is this xkcd sentence grammatically correct?

It's perfectly fine. Let's rewrite it to see what's going on.

Original:

You're in a box on wheels hurtling along several times faster than evolution could possibly have prepared you to go.

"box on wheels" => "car"
"hurtling along" => "going"

Creates:

You're in a car going several times faster than evolution could possibly have prepared you to go.

Now we can erase this unnecessary part:

You're in a car going several times faster than evolution could possibly have prepared you to go.

Finally creating:

You're in a car going faster than evolution could possibly have prepared you to go.

If you want to go even further, you can simplify all this to:

You're going faster than you are prepared to go.

Seems all fine here!

I think it would be better phrased as:

You're in a box on wheels hurtling along several times faster than evolution could have possibly prepared you to go. (Next 5 miles.)