I have a similar problem. Reads 60, doing 45. But this is a stock car (except for the go fast speedo mod!). Rear tires are 265/50/r15.
I am reluctant to remove the speedo housing with out learning more, I read somewhere in the archives about a shaft dropping inside the ZF if you weren't careful!
I have seen gear reduction speedo boxes, this was going to be my fix. Let us know how the problem resolves for you.
When I fix the high speedo, I will have to invest in a stroked, supercharged, 400M with Nitrous to get my Zero to sixty time back to 4.1 secs
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