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Discussion Starter #1
Hello,

My speedometer read fast (e.g. when going 60, it read 90), so I changed the 21 tooth speedometer gear to a 23 tooth. Nothing changed. The speedometer still reads the same as when I had the 21 tooth gear.

Any ideas about what else might be wrong? I'm left scratching my head. Any help is appreciated.

FYI, this is a C4, 8 drive gear teeth, 391 rear end ratio, tire diameter - 27.05 in.
 

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Do you have a gps available you can use to calculate your actual speed vs. the speedometer speed?
 

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Look for a post that Royce had on here - he had an excellent breakdown of which color gear (number of teeth) gave what ratio under given conditions. I believe he also had a stack of various gears. I'm surprised you din't see some change though...?
 

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Discussion Starter #5
Look for a post that Royce had on here - he had an excellent breakdown of which color gear (number of teeth) gave what ratio under given conditions. I believe he also had a stack of various gears. I'm surprised you din't see some change though...?
Yeah, I read through that post and a bunch of other posts/articles about gear calculation etc. I didn't expect 100% accuracy, but I expected my speed to read closer to actual speed. But really, there was NO change after switching gears. Still off by exactly the same amount. I'm wondering if a different gear did nothing, what I can do.
 

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Good question - IDK but I'm curious as to what you find since I haven't bothered finding the one for my AOD either - it's off about 12-15 mph
 

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Doe the speedo act normally other than being off? If it acts strangely, i.e., jumps around, the problem is with the speedo itself, the coupling mechanism specifically.
 

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Discussion Starter #8
Doe the speedo act normally other than being off? If it acts strangely, i.e., jumps around, the problem is with the speedo itself, the coupling mechanism specifically.
no bouncing or anything else I see strange. At 30 mph actual speed the speedo reads 40 mph, at 45 actual it reads 60, at 60 - 85, at 80 - 120.
 

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take it to a speedo/truck shop---they can build a gearbox that plugs into your speedo gear and put it spot on within 1/2 mph
doctordesoto
 

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Discussion Starter #10
I'll put this back on my todo list and will try a speedometer shop when I get tired of looking at the thing again. Thanks for the suggestions.
 

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Must be the twilight zone as I can't see how its possible no change occurred when you changed from a 21 to a 23. I know on older GM's each driven gear tooth added or removed was approx 3 mph.

Bill
 

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With gears having ratios and all, the error should always be a fixed *percentage* of miles per hour, no? This is what logic would seem to dictate and is also what I've observed with both my Cougar (speedo always indicates 20% lower than actual speed) and my Bronco (speedo always indicates 20% higher than actual speed). If your speedometer isn't off by a fixed percentage, doesn't that indicate that in some way the system is actually broken, not just a problem with incorrect number of teeth?
 

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no bouncing or anything else I see strange.

At 30 mph actual speed the speedo reads 40 mph [[33% high]]
at 45 actual it reads 60 [[33% high]],
at 60 - 85 [[42% high]],
at 80 - 120 [[50% high]].
Shouldn't that be a fixed percentage error in a functioning system?
 

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I think it is likely a satanic thing and cannot be repaired....
 

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If you could easily change the drive gear then yes you would use one with less teeth. The driven gear is the one normally replaced so you add teeth to slow the speedo down.
 
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