So today was science and physics day in the garage.... ya right.
I am putting an 18" front wheel on this build, replacing the stock 19". Having a smaller front wheel will throw of the reading of the speedometer and odometer because they are calibrated for the larger 19" wheel. It was suggested to me to get a speedo drive from a 400F because they come stock with an 18" front wheel. I was lucky and found one on Ebay in great shape. Its from a 1975 CB400F. The seller on Ebay even had a picture of the bike it came off of so I am "pretty sure" its from a 400F. You know how that goes. Anyway, I took apart the 400F speedo drive and the 750K speedo drive to see if I could find any difference in the gears. I didn't do any measuring but its was pretty obvious there are some differences. First is the part number. The part numbers for a 1975 CB400F and a 1975 CB750K are different so that's the first good sign. Visually comparing the speedo drives side by side they are identical however. The differences lie inside the drives themselves.
The internals upon first inspection look identical
The 750 gear on the right has tighter gearing
Looking from the top you can see the 400F gear has larger teeth
Larger gearing on the 400F here as well
I cleaned everything up and reassembled them with new grease. I used my speedo drive adapter to spin them one complete revolution and measured how far the speedo drive cable turned. The 400f turned about 2 3/4 revolutions and the 750K turned about 2 7/8 revolutions. I am guessing thats all it takes to make the difference in calibration. Hopefully and I am right and my speedometer will read correctly.
Any comment's, criticism, and experience on my methods here are welcome.