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So I have installed this C6 into my 1965 Gal and upon the road test the speedo is off by 10 mph. GPS / satellite tracking shows me at 55 and the speedo is saying i'm doing 65. I know you can change the speedo gear in the trans, but the question is which one do i get? My info tag on the door shows my rear end having 3:05 rear gears. I'm running 245/60/15 tires on 8 in wide rear rims. to me having larger tires would make the odometer show slower not faster. any ideas on what I need to get for a gear? also, is it just the 1 gear in the trans that i need to change ?
 

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Actually you'll need more teeth on the speedo gear to bring the speedo reading down, and 3 teeth is probably the right number but there's a bit more to it than that. You can also change the gear the drives the speedo gear to get ratios that you wouldn't be able to make by simply changing one or the other. Here's a nice page I found on the subject, unfortunately for me, there's no AOD on that page. =/



Speedo Gears
 

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It's all just ratios, and to bring down your speedo reading, you need to drive a gear with more teeth, not less. Let's say for instance you're right in the middle of the chart I linked so you have 19 driven by 8 for a 2.375 ratio. You're going 55 but seeing 65 on the speedo so you need to adjust down by 65/55 = 1.1818. 1.1818*2.375=2.8068. You could use 23/8 or 20/7 to get that ratio (as an example only. You'll have to see what you actually have in there first)
 
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