I recently rebuilt the original BOP rear end in my 1967 Coupe. The rear end is original to the car. I also changed the tires from F70 14 inch tires and wheels to 15 inch 215/65/15 tires and wheels. Based on my research, the new and old tires and wheels both have a diameter around 26 inches. The car had the factory positrac unit with 3.23 gear ratio. I confirmed that when I removed the gears and the posiunit. ince there is not a source for new 3.23 gears, I installed a new posi unit with 3.55 gears. The old gears were very sloppy and noisy. That problem is solved. Before I did the work, I used a digital hand held odometer/speedometer application on my phone and compared that to the the speedometer and odometer readings in the car. The readings were very close (within 1%.) The driven gear for the speedometer that was in the car was the red gear with 21 teeth. I was able to put my scope in the transmission and confirm that the drive gear is the neutral color with 8 teeth. When I ran the original calculations under the original set-up, I came up with the 8 tooth drive gear and 21 tooth driven gear. This also matches what is in the 1967 parts and service manual. I do not think the tire and wheel change is having a significant impact on the speedo or odometer since the size of the old and new are so close. I knew that changing the gears from 3.23 to 3.55 was going to impact the speedo and odometer accuracy, so I did the calculations to see if a driven gear change would put me pretty close to accurate. That calculation showed that I would need a driven gear with 22 teeth so I installed that gear at the same time that I did the work on the rear end. The speedometer and odometer are 5% off at 60 MPH. The speedometer shows that I am doing 60MPH when the digital speedometer shows 63 MPH. The odometer over a 25 mile drive shows that I am off about 3-4 %. I tried to keep the speed at a constant 60 MPH to get the odometer comparison but I am sure my speed probably was not consistently 60 throughout the 25 mile drive. I am more concerned about the speedometer than the odometer. The speedo fix may bring the odometer in line anyway. Since I was pretty sure that I had done the calculations correctly, I went to the firebird parts list for the Saginaw 3 Speed manual tranny and found that there was a Speedometer Driven Gear Adapter (part # 9775436) with 20 teeth that was used with the 3.55 ratio in 1967 3 speed tranny. I am sure that part will likely fix my issue. However, I have not been able to find any other information about this adaptor. I could not find it in any of the illustrations in the parts or service manuals and my search of the web have all come up empty. First question. Does anyone have any information about the adaptor including how and where it is installed and if that part is available either as a repro, OEM, or aftermarket? Second questions, if I am doing my calculations correctly, I would be able to install the original 21 tooth red driven gears. To do this calculation, I simply took 22 (teeth on the current driven gear) times 95% (the speedo is off 5% at 60 MPH) to come up with 20.9 teeth which would be close enough if I installed the 21 tooth gear. Could someone confirm this will work or if you have other suggestions I am open to those as well. I have also attached the page from the parts manual that shows the adaptor, drive gear, and driven gear info.