Counting the Miles, But Not Too Many

I also spent a little time experimenting with my speedometer.  I have a 3.90 differential to rebuild and I definitely plan to use it.  However, it changes the gear ratio (from 4.22) and therefore the speedo won't clock properly.  I bought a speedo for a later car that works with that ratio, but of course it is smaller and has the trip reset out the back instead of the bottom.  I also have 3 or 4 "correct" extra speedometers (the "good" one is tucked away just in case).  So I figured, why not swap insides with one of those?

Hah.

Not so simple.  There are just enough differences between the two that it's not a direct fit.  By the time I was done screwing around, I had taken both speedos completely apart.  I finally figured out that one important piece is the same--the base.  The base for the newer speedo also has the correct ratio gear for the odometer and the screw holes for attaching stuff line up the same.  But the disc and shaft for the newer speedo (the part that moves the needle with the magnet) only works with that base, and the spring on that one is attached differently than the original.

After staring at it for a while, I figured I had gone too far to turn back.  After a beer for good luck, I teased the springs off the old and new speedo shafts and swapped them.  I actually managed to do this without breaking the springs or the shafts.  I put the new disc with the old spring back on the new base with the old odo/trip assemblies and trip reset.  It all fit, and spinning the input shaft makes the needle move and works the levers that actuate the odometer and trip meter.  It's not exact because the new shaft is a little different, and the adjustment for the old shaft doesn't work too well, but it moves and returns to 0 like it is supposed to and doesn't bind up.

Theoretically, this should work with the 3.90 diff.  If it doesn't, then I'll resort to something else I wanted to try--a GPS speedo input.  There's a company that sells an electrical-to-mechanical drive kit, and another that sells a pulse driven counter attachment where the cable attaches to the transmission.  So I could make that work too, and it would probably work better--but I like my one-off speedo and I really hope it works properly.  

I should have taken pictures because it was a fascinating job and I learned a ton about how the speedometers in older cars work.  Maybe next time.