Speedo is wrong after dif swap. yellow box or gear change?

I replaced my 4.3 open dif this weekend with a 3.9 Torsen.:D I have a spare 6sp trans I pulled the electronic speedo from to put in my 5sp. I swapped only the gear onto the 5sp and when I drove I had no speedometer movement. My guess is that the 6sp gear is too small to mesh with the 5sp internally. I swapped it back and, the speedometer workes but it is not near correct and requires me to perform math when I see The Man approaching. I want to stay with the 5sp/3.9 combo and not use the 6sp. I'll be looking to trade it with someone for a good 5sp sometime. Is there anything I can do with what I have to get a working speedometer? I have seen a 'yellow box' advertised but before $125 I would like to know what ohers have done that have this combo.
Thanks.

Might not be a bad thing. You used to be able to get out of speeding tickets if you could show your speedo was off.




























Umm I'm not referring to banana hammocks either. ::Banana::

I know I've seen relevant info recently on miataturbo.net. Just a matter of digging it up!

There is a difference in spacer length on the speedo drive gear between the 6-speed and 5-speed, thus the lack of signal to your dash. If you can get that spacer from your 5-speed drive gear onto the one from the 6-speed, you might just have a winning combination!

Thanks.