700R4 speedometer question?

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Ok you guys with the 700R4s or 2004Rs? How do you connect the speedometer up to the
overdrive trannies? If I'm not mistaken, they all have an electric speedometer outputs, right?

I'd really like to have an overdrive, but if I have to spend $300 dollars for a computer to read my
speed, I'll have to rethink this. I want to keep the original mechanical speedometer.

I've searched all 35 pages of posts here but couldn't find any information about how to
handle this issue, except for the $300 dollar computer. But I've thought I'd seen some type
of a converter mentioned before.

Thanks.