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Old 09-27-2005, 09:07 AM
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What speedo gear?

Just put 4.11's in my nova with the TH350 tranny and 8.2 rear end. Anyone know what speedo gear I will need or where i can find out?

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Old 09-27-2005, 09:21 AM
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I've seen other posts by kinda confused I have read a lot of different gear suggestions. I have 26" overall tire height. Right now my speedometer reads 50mph when I'm going 30mph (20 off). I knwo b/c my town has those damn radar boxes the cops post in different places.
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Old 09-27-2005, 08:56 PM
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Look for a speedo shop and have them install the magic box where your speedo cable goes into your tranny. This will recalibrate it for you.
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Old 09-27-2005, 09:36 PM
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Calculation method

As suggested, you may have to install a ratio correcting adapter.

On smaller differences, you can use this logic:

New Ratio (4.11) / Old Ratio (3.08?) = 1.334

Now multiply this by the number of teeth on the driven gear (17?)
1.334 x 17 = 22.67 which says that you need a speedo driven gear with 22 or 23 teeth. (More teeth on the driven gear will "slow down" your speedometer)

Similarly:
Displayed Speed (50)/ Actual speed (30) = 1.666
(Holy crap! What gears DID you used to have? 2.48?)
1.666 x 17 = 28 teeth ... which I doubt they make.

Hmmm ... 50/30 is very similar to the metric conversion factor, as 50KMH = 31.25 MPH. Perhaps you could scrounge a ratio adapter from a Canadian vehicle?
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Old 09-28-2005, 06:00 AM
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I used to have 2.56's. The pinion gear was about the size of my head,lol.
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Old 09-28-2005, 06:13 AM
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Quote:
Originally Posted by 1quiknova
Just put 4.11's in my nova with the TH350 tranny and 8.2 rear end. Anyone know what speedo gear I will need or where i can find out?
According to my calculations you will need a 7-tooth (pink) drive gear, and either a 21-tooth (red) driven gear, or a 22-tooth (gray) driven gear, or any other combination of gears that equal a division of around 3.18 (+ .5).
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Old 09-28-2005, 06:45 AM
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Quote:
Originally Posted by M&M CUSTOM
According to my calculations you will need a 7-tooth (pink) drive gear, and either a 21-tooth (red) driven gear, or a 22-tooth (gray) driven gear, or any other combination of gears that equal a division of around 3.18 (+ .5).
I'd be interested in knowing how you calculated that... excellent work!

I'm still thinking Ford C6 when I made my suggestion. The number of drive gear teeth is not easy to change on a C6, as they're cut right into the output shaft!
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Old 09-28-2005, 06:55 AM
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Years of wisdom, knowledge, and really-really good guessing, led me to several websites I have linked within my Information sticky threads.
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Old 10-01-2005, 08:33 PM
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http://novaresource.org/speedo.htm is a good one.
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Old 10-03-2005, 06:24 AM
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Yup, that's one of them, also at the Novaresource site is the page 2 of that stuff covering the gears in numeric order and by size whether the Chevy TH-350 or the BOPC TH-350.

The other one is http://novaresource.org/speedo2.htm

I have links to several in the Information threads and wanted to include ALL of them in my speedometer thread, but a lack of time has just postponed it.

If you happen to read through some more of jimfulco's posts, he really knows his stuff around here too.
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Old 10-04-2005, 09:15 AM
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I just talked with a local speedo shop that has been around forever and they said that I will need the magic recalibration box.
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Old 10-04-2005, 09:18 PM
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Which they will happily sell you for $80 or so. With 4.11's & 26" tires (speedo gear ratio = 3.19), you could use a 7-tooth drive gear & a 22-tooth driven gear (ratio = 3.14) and your speedo would read 61mph at a true 60mph. I could live with that small an error for $80.
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Old 10-05-2005, 08:13 AM
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If I install the gears don't I have to remove the tailshaft to replace one of the gears? And is there more involved than unbolting the tailshaft I haven't taken a tranny apart before besides removing the valvebody and installing shift kit
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Old 10-06-2005, 06:48 PM
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Remove the driveshaft, unscrew the speedometer cable, unbolt the tailshaft housing, and press the retainer clip holding the drive gear on the shaft down to allow the gear to be slid off the tailshaft (it's usually difficult to slide the gear, but it will move with carefully utilized moderate force).

When I changed mine I replaced the tailshaft housing seal, and slip-yoke seal.

The whole thing took about an hour, and I only had to empty about a half of a dixie cup of fluid out of the catch pan.
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Old 10-06-2005, 06:54 PM
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Quote:
Originally Posted by M&M CUSTOM

The whole thing took about an hour, and I only had to empty about a half of a dixie cup of fluid out of the catch pan.

But be sure to have the rear of the car elevated .....so the fluid goes the the front...

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