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Old 08-08-2002, 11:40 AM
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Post tire size and speedometer accuracy

I have R14s now and am considering changing tires. How much will this mess up my speedometer reading?

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Old 08-08-2002, 11:50 AM
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Put on smaller tires and your speedometer will tell you that you're going faster than you actually are. Conversely, put larger tires on it and the speedometer will tell you you're going slower than you actually are. How much of a change there is will depend on how much of a size difference there is between what you're driving on now and what you replace them with. Speedometer correction can be attained by replacing the gear in the speedometer drive in your transmission.
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Old 08-08-2002, 12:07 PM
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On my ford I ran along side of someone to figure how far off my speedo was or you could calculate it. After I figured out how far off I was I removed the speedometer cable and gear from the trans, counted the teeth and then figured how many teeth I had to add. On my ford each tooth added to the gear dropped my speedo about 4mph. Also the fact that you have 14's mean nothing, the diameter is what's important. You can switch from 14 inch to 15 inch rims and still maintain about the same total diameter depending on the tires profile.

[ August 08, 2002: Message edited by: dmorris1200 ]</p>
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Old 08-09-2002, 01:27 AM
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There's a handy sizing calculator here;

<a href="http://www.tirerack.com/index.jsp" target="_blank">http://www.tirerack.com/index.jsp</a>
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Old 08-09-2002, 06:51 AM
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thanks guys! Yeah, I realized afterward that I didn't give the whole tire size. oops.
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Old 08-09-2002, 11:56 AM
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Way out....I understood that and that's why I gave a generic answer....
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Old 08-09-2002, 07:12 PM
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Yea well I work with customers that come in and try and tell me their stock 1.6-4 cyl. Nissan Sentras with their goofy ground effects packages are running 13 second quarter miles (yea right!). So I tend to not assume or 'understand' anything. It's just the way I am.
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Old 08-10-2002, 05:27 AM
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maybe you should tell them that quarter mile times are *usually* timed from a 0 mph starting point? naw!
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Old 08-10-2002, 06:32 AM
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That's a good one, I'll have to remember that.
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Old 10-17-2003, 08:36 PM
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speedometer gears

i was just wondering if you guys can help me. what is the speedometer drive gear teeth on an 1986 trans am, 6,7 or 8?:


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Old 10-18-2003, 01:38 AM
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You have to take them out and count them, then you tell use.

Of course you could look at the end of the sender gear, and see what color it is, then go to the GM dealer ship, they should be able to tell you.

Troy.
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Old 10-18-2003, 08:47 AM
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where is that calculator link? I need a speedo gear for a 700R4 I am running 32" tall tires, and a 4.30 gear. anybody have a part #???? thanks..... LUKE
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Old 10-19-2003, 02:50 PM
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thanks a lot troy i will be sure to tell you when i find out myself.
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