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Old 11-30-2013, 11:54 PM   #1
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Region: Texas
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Speedometer gear

I'm trying to figure out which gear I should have in my c4 to get an accurate speedometer reading. C4 tranny with a 3.50 gear in the rear end and 235x60x15 tires. Not sure which gear is in there now but it's about 10 or so mph's off. It's on the "to do" list for this winter. Thanks.
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Old 12-01-2013, 03:55 AM   #2
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What transmission does your car have? Here is a good calculator. West Coast Classic Cougar Speedometer Gear Calculator

Just put in 3.50 (your rear ratio), 26.1 (your tire's outer diameter) and find out how many teeth the drive gear (the gear mounted on the tailshaft) has. I put in 8 (that was the standard for the C4) automatic, and it said you should use a 21 tooth gear (that's the closest to what you would need to be 100% accurate). With that gear your speedometer would read 1mph fast. But that's if you have a C4 with 8 teeth on the drive gear.

Drive gear=gear mounted on tailshaft, Driven gear=gear mounted on speedometer cable
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