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  #1  
Old 11-03-2002, 06:09 PM
pro-street dart pro-street dart is offline
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Default Speedometer Question???

I now have 31" tires with 4.88 gears on my dart - which will no doubt throw off the speedometer, what speedometer gear would I have to get in order to get it to work close to what it did originally?? Any help appreciated.
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Old 11-03-2002, 07:47 PM
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ehostler ehostler is offline
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If I've done the math right, you will need the 2538941 (41 tooth) gear.
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Old 11-03-2002, 08:00 PM
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ehostler ehostler is offline
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I have redone my math and confirmed that you would need approx 41.426 teeth on the gear, to be accurate. If you go with 41 teeth, your displayed speed will be just a little faster than actual.

My base numbers are a 4.56 gear with a 30" tire uses a 40 tooth gear. I got this from http://www.moparts.org/moparts/tech/pages/speedo.html
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Old 11-04-2002, 07:35 AM
pro-street dart pro-street dart is offline
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Thanks for the info!!
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Old 11-04-2002, 01:39 PM
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TOOL MAN TOOL MAN is offline
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Default eholster

Eholster,

thanks for the info showing all the #'s for the ratios. i asked this question some time ago and the only answer i got was to call mopar performance. this helps

thanks again

tool man
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Old 11-04-2002, 03:56 PM
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ehostler ehostler is offline
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Glad to have been of help.

I may have seen the earlier request, however I forgot that I had this bookmarked, until I recently saw it, while skimming my bookmarks.
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Old 11-05-2002, 11:10 PM
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ehostler ehostler is offline
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For any of you that want the math, to calculate the correct speedo pinion, here it is -

((63360/tire circumference)*gear ratio)/76.3 = number of speedometer gear teeth
tire circumference = tire diameter * PI(3.14159)

you should round the number of teeth to the nearest whole number. If you round down, the speedo will read a little fast. If you round up, the speed will read a little slow.

PN for speedo gears are 25389XX (where XX=number of teeth)

I set this up in a spreadsheet and I just have to punch in the rear ratio and the tire diameter. it then gives me the correct number of teeth.
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