In most cases when the second worm gear is defective and in need of replacement the cause goes un-repaired. The W series speedometer heads had a common issue in them in the form of the first worm and magnet bushing.
The first worm and magnet bushing was made from what is called oylite and what will cause the first worm bushing to fail prematurely is one of the most common mistakes made on almost all cars. Customers often get a speedometer or tachometer cable that will make noise, jump or for what ever other reason, the cable is pulled and greased with any basic type of lubricant. The issue is the cables are reverse wound and if you lube the entire cable this grease will be pulled into the head. Combined with the fact that Oylite doesn’t like grease and the fact that the magnet head attracts lint, dust and other flying particles to the head, the combination is a fast track to a failure. If you go to this link you’ll see the proper way to lubricate a speedometer or tachometer cable.
Over time this bushing will wear out and cause a miss-alignment condition between the first worm (A) and the second worm (B). (The second worm drives the odometer assembly). When the second worm wears out as shown in the right hand side of the picture, the speedometer keeps working but the odometer will stop turning.
What generally happens is the second worm starts wearing out (at point B) and when it came to the stress of turning all the numbers on the odometer wheel it finally tanked. This in turn is causing stress on the first worm (which drives the speed cup the needle is attached to) at Point A and B and is causing it to bind and jump.
Originally the speedometers had a metal gear and when the first worm bushing failed both the first and second worm gears would be destroyed, later replacement speedometer head however came with a plastic gear. This plastic gear was actually a good thing because if the gear failed it would strip out but not take out the first worm and magnet. The new reproduction gears do not resemble the original gears in a appearance but functionally they work fine.