2600 Spider speedometer error

I recently decided to fix my speedometer error. According to road tests by Road & Track and Car and Driver magazines done on new cars in the early 1960s, these cars showed errors of about 4% fast. My car has 15" Borrani wire wheels with 185/70 tires which results in a smaller diameter than the originals, so my error is much greater. When I finally got my car back on the road after restoration, I calculated the actual speeds relative to engine speed and just used the tachometer to easily determine road speed in 5th gear. My figures show the speedometer to be about 11% fast, confirmed by using a GPS.

I never considered sending my gauge to one of the well-known shops like North Hollywood or Palo Alto, but I have read that they are no longer accepting Veglia instruments for re-calibration anyway. After a little research, I learned that there are speedometer "ratio adapters" which fit between the speedometer drive on the transmission and the cable. These little gearboxes change the output speed of the speedometer drive. I decided that there should be just enough room to mount one. The problem is there are none with metric threads. The standard thread is 7/8" - 18. The speedometer drive on the Alfa transmission has 20 mm X 1.0 thread where the cable attaches. For fun and therapy, I decided to try to make thread adapters to allow fitting a ratio box to my car. I found a used ratio adapter on eBay that had a .8974 ratio, just what I needed.

I have a vintage 1939 craftsman 6" lathe and just enough knowledge and experience to get me in trouble. After several hours of play, I had a threaded bush with 20 mm internal thread and 7/8" external thread which allows fitting the ratio box to the speedo drive at the transmission. The difference in diameter between the two threads is just enough to make a threaded bush work. However the OUTPUT of the ratio box has a 7/8" male thread and has to be stepped down to a 20 mm thread. This creates a problem because the inner speedo cable will not reach the extra 1 ¼" through this new thread adapter. I used the milling attachment on my little lathe to make a .104" square on one end of a round bar and drilled the other end to accept the inner speedo cable. This end then had to be crimped to capture the .104" square of the cable. (The .104" square is a long time standard for these cables.) This little piece acts as an extension for the inner cable.

I will post a few pictures which should clarify all this. Wow, I just realized that with a few more words, this post would be longer than one of Jay's!

Larry Bono