Have you considered making it larger? going back and doing your other graphics might be a bitch, but it'll be much easier and look much better in the end. There's kind of a sweet spot for every sprite that seems to lend to it perfectly, where your clusters don't just look like noise, and you can fit in all the detail you want.
If not, crits on the current one. Arms way too long, hands and feet too big (one is bigger than the other?). (our) Left foot could be rotated towards us a little more. Head is neckless/chin too low. Still no outline on the headband, maybe get rid of it altogether? unless its really important the extra space could be useful for facial features. One of the legs is the wrong width, I'd suggest widening the right one. Arm connects to shoulder weirdly, move it back, rotate the shoulder. torso could maybe be a pixel longer. Stance seems impractical if he intends to start moving. Lighten the outline of the left knee pad. You should move the closest arm back a bit, seeing as the angle of the shoulders will distance the two arms.
I like the knee pads/shoulder pads, they're actually a useful design choice when trying to show the direction of the limbs.
The car. Firstly, the wheels aren't round. But I really have to ask, what's going on with those segmented grey bits along the top of it? There's banding around most of the outlines, now would be a good time to employ some AA.
Lastly, your ramps overall are quite straight and boring, play around with some hue shifting, read up on some colour theory (there's great thread
here)
One more question, actually, do you start with line art, or a silhouette? try just drawing the silhouette in a few different colours for different parts, then go over with your tones, or if your uncomfortable with that, you can go over the silhouette with line art. It's just a lot easier to play around with a few blobs of colour than with lines.