Algodoo

I intended this beauty for the actual Baja contest, but the contest closed before I got around to uploading it. Now it's slightly improved for the collab.

The soft wheels are reeeaall simple. Make a small wheel, then clone it and scale up the new wheel, then hinge them together in two to four places, then make the larger wheel super light.
This approach is not as true to actual softbody behavior as most other kinds of soft wheel, but it doesn't lag, can't go flat, has no small parts to stick in polygons, and doesn't malfunction at high speeds.
The rest of the suspension is pretty cool too, but I'll let you piece apart how it works on your own.

CONTROLS:
Arrow keys to move, UP to run the super-booster. Not recommended for most of the track, and do keep in mind that it will turn the front down when used in midair. It's most useful for helping with the big hill or random troublesome obstacles that the wheels aren't powerful enough for on their own. If you use the booster for extra speed, let go of the wheel controls because they can only slow you down above thirteen m/s.