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only way i know is to change out the spedo gear in the tranny... really easy... are you over or under?? I got the same prob.. i'm 10 under the actual speed.. i have a gear for a 3.55 rearend then had to change to a 2.94 rearend... so thats why i'm off.
 

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yep, pinion gear in the tranny....now if anyone has an idea how many teeth that thing has stock and what pinion gearing gives what sort of correction ???
 

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What else did you change besides the cable? The cable itself has no effect on the registered speed, that's what the driven gear in the tranny is for.. There's different tooth counts depending on your gears and tire size.

Matt {peace}
 

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TaZMaNiaK said:
There's different tooth counts depending on your gears and tire size.
Got a list that is for tires larger than 30"? I only found tire sizes between 26" and 30", though I can derive from that what the correct number would be for my tire size-gear combination
 

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i rememver someone showing a formula on here how to figure out how much ya need to do to correct it.
 

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well, I know a formula:

new tire size x new pinion gear = old tire size x old pinion gear

I was just curious if there was a way to get the new pinion gearing w/o having to pull my current one first :)
 

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well, I know a formula:

new tire size x new pinion gear = old tire size x old pinion gear

I was just curious if there was a way to get the new pinion gearing w/o having to pull my current one first :)
 

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well, I know a formula:

new tire size x new pinion gear = old tire size x old pinion gear

I was just curious if there was a way to get the new pinion gearing w/o having to pull my current one first :)
 

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santak said:
well, I know a formula:

new tire size x new pinion gear = old tire size x old pinion gear

I was just curious if there was a way to get the new pinion gearing w/o having to pull my current one first :)
If you email me your snail mail address, I will send you a copy of the parts book that shows all the tire sizes and pinions. This list will be from a 86-87 parts book but should be the same for all years from the early 80's on up through the early 90's. There are too many pages to email. ;D
 

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Ok. My speedometer was off by 25%. meaning when I drove 125 miles, my odometer showed 100. my speedo was reading extremely slow, too (obviously). So I went to a local shop, told them I needed a gear box to speed up my speedo 25% and in about 20 minutes they made me one. Cost me $40. Fixed and got it on the gnat's ass. So, if you will figure out what % you are off, I can get you one of these made up. Use the milemarkers on the highway as a base for your measurement. if you have a trip odo, that is easiest. drive 10 miles and see what your trip meter shows. or 100. just make sure you drive in increments of 10 miles 'cause it makes it easier to figure.

Your speedo shouldn't be exactly 10 mph off thru out its range. it's inaccuracy is increased as your speed increases.
 

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One tooth on the gear is about 3 mph. First, you have to figure out the RPM of the driveshaft..
65 X gear ratio X 336 / tire diameter
example 65 X 3.9 X 336 / 33 = 2581
then you take that number and devide it by 78 to get the number of teeth on the speedo gear.
example 2581 / 78 = 33
 
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