My speedometer is showing about 10% fast. I'm assuming I need to change the spped-o gear in the transmission but how do I know what to change it to???

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85T/A 305/QJ work in progress.

 

Mine was off on a 86 camaro I used to have and what I did was just go to a highway and find mile markers, stop at one, then on your mileage, look at the 12345.X, see what the X is at then go to the next mile marker and see what it is at then, then just go to a place and have them make you a box that would adjust it, it should just fit itbetween your speedo cable and tranny, at least on a cable driven one, i'm not sure about electronic ones, just tell them it turns .9 for ever mile or something. They will be able to fix you up. Hope i explained this good enough, sorry its so long.