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Old 08-12-2006, 12:32 PM
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speedometer recalibration

I have changed tire size,and gear ratio on my 86 t-bird. I know the % of error, how do I change speedo readout. There is a box on the side of tail shaft where the old cable drives used to be, but there is only 2 wires coming out. Is there a gear in there I can change? 86 Tbird w/ aod trans . thanx

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Old 08-12-2006, 01:40 PM
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On the Chevrolets with electric speedometers ... you can change the chip prongs up and down combinations and get the results ...

Maybe the same on a Ford ... I do not know ...
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Old 08-12-2006, 11:13 PM
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speedo calibration

A Ford fix like a GM? that would be nice for a change.
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Old 08-13-2006, 08:46 PM
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Yes, there is a gear in there. If the speedo's reading too fast, you need to use a driven gear with more teeth. If it's too slow, use less teeth. Count the teeth on the one you have, and multiply that number by the fraction made by your indicated speed/true speed to see how many teeth you need.

Unlike with an old Chevrolet, the drive gear on an AOD is not removable, it is machined into the tailshaft, so you can't change it. Ford only makes AOD driven gears up to 21 teeth, so you may not be able to get it exactly where you want it. Aftermarket suppliers of 5.0 Mustang stuff sell a 23-tooth gear that fits, sort of, but since it's some kind of Jeep part or something, it supposedly tends to wear out quickly.

If your calculation ends up between whole numbers, the higher-numbered gear will make your speedo read a bit slower than you're really going, which might get you in trouble with the law.

What gears & tires did you use?
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Old 08-14-2006, 07:51 PM
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speedo calibration

I don't know what the original gears but I scored a 9


I"m not sure what the original gears were, but the new housing I got is a 9 inch that has a 3.90 gearset. I've gone from 26 1/2 " tall tires to 29" tall and the speedo definitly reads slow. 64 is around 70 according to the odometer difference between mile markers on a straight Interstate section for 5 miles. Thanx, I'll be able to compute the proper gear due to the shared info. Thanx again.
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Old 08-14-2006, 10:51 PM
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http://novaresource.org/speedo.htm has a formula you can use with your gear ratio & tire diameter

http://tciauto.com/tech_info/speedo_gears.htm has some calculators
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