Which speedo gear?

[07GtS197]

This is for my 87 Grand Marquis. Right now it has 4.10s and stock hight rear tires. I’ve run it through the calculators a few times and get a 21 tooth gear but its still 20% over the actual speed. The combo is an aod, 215 70r 15 tires and a 21 tooth speedo gear. It would be close to a fox with 28” tires, I don’t see fox guys having an issue so why do I? Am I doing something wrong?


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[Silver95bird]

I recall something about there being 2 different output shafts. one with a 7 tooth gear on the shaft, and one being an 8 tooth. If a 21 tooth driven gear works in a mustang but not in that car, it may be the reason why.

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[07GtS197]

I’m starting to suspect the same. Maybe I’ll try the 23 tooth gear for fun, maybe it’ll actually work.


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