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What is the correct size spedo gear I need. I've got 89 tranny, 3.73s, original pony wheels,Steeda says red 21 tooth, Dallas Mustang says White 23 tooth. What to do, what to do. I had 23 White that I pulled out, but It seemed that my car was reading a little off, maybe just me. Any info or suggestions?
 

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During the '89 model year, the speedometer drive gear (on the output shaft) went from having seven teeth to eight. You have to either get the info from the transmission tag OR count the number of teeth on the drive gear.

And what size is the rear tire?
 

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Mine is a yellow 7 tooth. If you go by some charts I've found online, the 84-89 are 7 tooth and you need a red 21 tooth to recalibrate correctly. IT says that 90-93 have a 8 tooth drive gear and you need a white 23 tooth. Dallas mustang says they think I need the white one, which the chart says it's for 4.10s, but that's just one guy who could have screwed me up, but then again I pulled out a white one. Maybe people before me have got it wrong too. I was told I have 3.73s, but I dont' think they are 4.10s. Heres a chart to look at.

http://www.latemodelrestoration.com/lrs.aspx?ref=mu&gotopage=/content_50_home.aspx
 

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Drive Gear x Rear Gear x Tire RPM / 1000 = Driven Gear

7 x 3.73 x ??? / 1000 = ??

FoxChassis said:
what size is the rear tire?
 
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