How much would speedo be off by?

Moy

It's broken...
Aug 6, 2008
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Beach Park, IL
I've searched, and found a thread with a post saying that a member had a N/A differential (4.30) with an 87-88 turbo 5spd driven gear (for 3.91) and that it was 11mph off at 65mph.

What I need to know is: how far off, percentage wise, would my speedometer be off by, if I kept my 4.30 differential with the 3.91 driven gear?

I have a 3.91 in my garage, but I cannot for the life of me get the cover plate to sit flush, nor can I get the bolts to thread all the way in.

If I have to, I will be keeping my 4.30, which is why I asked.

Thanks,
Chris