Speedometer is driving me crazy- Something is Wrong

Sorry to hijack but is there more than 1 worm gear for the 727's? Reason I ask is I got a 39 tooth (green) gear to swap out whatever was in it and when I pulled the original gear out, it was quite a bit smaller in diameter (don't know how many tooth) than the new gear. Because of this, I didn't think the gear would fit and sure enough, I could not install the new gear.

Thanks in advance. Bill
Never heard that before, strange