Why don't you put the mechanical speedometer drive gear in your T5 and be done with it?
It's been done before, as seen here:
http://oldgmctrucks.infopop.cc/eve/forums/a/tpc/f/633004236/m/4141096923?r=5971090133#5971090133
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Seems the problem is the shaft doesn't have a place for it, but some epoxy solves that.
What do you think?

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-