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Question on gears

This is a discussion on Question on gears within the Firebird / WS6 forums, part of the Vehicle Specific category; My speedometer is off by around 15mph. It reads 60 when I'm doing 75. Checked it with my gps as ...

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    Member imprtkillr2810's Avatar
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    Question on gears

    My speedometer is off by around 15mph. It reads 60 when I'm doing 75. Checked it with my gps as well as the cobra iradar app on my phone.

    From what I've read it sounds like a gear issue. The car is an A4 and the build code says it has the 2:73 gears. It runs around 1700 rpm when the GPS reads 70 and the speedometer is around 55.

    I haven't been able to get the car up to count the rotations yet, but does this sound like a gear issue or could it just be the speedometer? Tires aren't stock size but according to Tire size calculator with the tires I have I should be at 60.7 when the speedometer reads 60. That's a long way from a 15 mph difference.

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    Spaz is My Mentor SMWS6TA's Avatar
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    It could be the little plastic gear in the transmission that the speedometer cable connects to. If it's bad you'll have messed up readings. This would be my first check.

    Even with 4.10 gears you would only be within 2-5 mph off not as much as you're reporting.

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    M6 King Hot Black Trans-Am's Avatar
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    The speedometer will be off by a percentage not a certain amount mph through the range. Granted I'm a M6, but I'm off by 20% going from 3.42 to 4.11. When the speedometer shows 85 I'm really at 65, I'm not running stock tire size either so that comes into play as well.

    With tire size and if previous owner did a gear swap to like a 3.30 sounds legit to me.

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    Member imprtkillr2810's Avatar
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    The tires (im assuming you mean rear) are 295/30/19. But as I said according to the tire size calculator it says "speedometer reading with non stock tire is 1.2% too slow. At 60mph you would actually be doing 60.7mph."

    Gears was the only other thing I could think of. I havent tried it very much at lower speeds as im usually just in traffic. I did notice on my way home the other day at around 43-44mph on the speedometer my gps was telling me 50, so I would say it is off my some percent. Most of the forums Ive read wanted to know the rpms at 70 thats why I mentioned that in my firs post and how far off it was.

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    Spaz is My Mentor SMWS6TA's Avatar
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    Quote Originally Posted by imprtkillr2810 View Post
    Most of the forums Ive read wanted to know the rpms at 70 thats why I mentioned that in my firs post and how far off it was.

    also what gear you happen to be in for M6 folks

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