Speedometer failed; Odometer works
 

The speedometer on my '95 318i fails sometimes. It will fall to zero when it fails, but the odometer continues to operate correctly all the time.

I believe that the problem is not the speed sensor in the differential because the odometer works fine. Is this a good diagnosis?

I found a site that offered replacment gears for speedometers for some BMW models, but it didn't specify which models so I was sceptical. What should I do?