I'm having the same problem...found 9/10, but no idea where the final one is. I've broken
every crate, tree, barrel, and car looking for the final one. It just doesn't seem to be
anywhere.

I managed to grab the key, but that 10th gear is driving me insane...

Can anyone help? Please?

I could have sworn there used to be a list online of all the gears, but I cannot seem to
find it for the life of me. At any rate, here's the ones I can remember:

-2 gears in boxes in the 'castle gate' area
-1 gear in a box in the missile area
-1 gear in the midair where the gas can was above the volcano
-1 gear in the middle of a circle on the ground by the big jump.
-1 gear in midair off the edge of a cliff just above the city.
-2 gears hiding in the woods
-2 gears in the city on the opposite side of the key: One off a ramp near the
curvy-roofed buildings, and the other right next to it across the way at the end of a
ledge.