If the right-most gear is turning freely, that's your problem. Fixing it is quite simple - I've attached an edited version of an earlier post that should be helpful in this regard. You may wish to review the rest of the thread using the search function.
A broken mechanical VDO odometer is usually the result of trying to reset the odo when the car is moving - this will loosen the right-most gear from the shaft. And it is this gear that drives the whole counter. So lesson one is that when you do get it fixed, reset your odo ONLY when at a stop.
Fortunately, you can easily fix it yourself by re-securing that right-hand wheel - either by scoring the shaft (as Benzmac suggests) or with an anaerobic adhesive. Unlike super glues and most other adhesives this type will not creep through the adjoining air space to the other gears. (You only want to glue the right-most one!)
I prefer the adhesive technique in terms of ease and durability, but be sure to use the appropriate product (Loctite Sleeve Retainer #620 is my choice).
When you do this, or any other job requiring removal of the instrument cluster, be careful not to jam or bend the speedo cable.