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Author Topic:   speedometer is wrong now...
68 Coop
Gearhead

Posts: 4243
From: Mesquite, NV. 89027 M&M# 4256
Registered: Oct 2004

posted 05-14-2006 10:57 PM     Click Here to See the Profile for 68 Coop   Click Here to Email 68 Coop     Edit/Delete Message   Reply w/Quote
that I have my new rear gears, and I was wondering what to do to correct it? I'm not totally sure, but I think it's off around 10mph now, and it seems to be fast. Thanks for any suggestions.

------------------
William
The easiest way to find something lost, is to buy a replacement.

68 Coupe
289
C4
3:55's/Trac-Lok

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68mustang351w
Gearhead

Posts: 500
From: San Jose Ca
Registered: Sep 2004

posted 05-14-2006 11:20 PM     Click Here to See the Profile for 68mustang351w   Click Here to Email 68mustang351w     Edit/Delete Message   Reply w/Quote
Check this link The drive gear should stay at 7 (if the original rear end ratio was between 2.70 and 3.50 OR 6 if ratio was between 3.50 and 4.30*) and you are solving for driven gear... David F.

* according to NPD catalog

edit to add statement above

[This message has been edited by 68mustang351w (edited 05-14-2006).]

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68 Coop
Gearhead

Posts: 4243
From: Mesquite, NV. 89027 M&M# 4256
Registered: Oct 2004

posted 05-15-2006 12:23 AM     Click Here to See the Profile for 68 Coop   Click Here to Email 68 Coop     Edit/Delete Message   Reply w/Quote
Thank you, that's very informative. I didn't realize it was that much to it.

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zieber
Gearhead

Posts: 102
From: Central Coast, CA, USA
Registered: Mar 2006

posted 05-15-2006 01:24 AM     Click Here to See the Profile for zieber   Click Here to Email zieber     Edit/Delete Message   Reply w/Quote
I don't know for sure if this calculator works but it is simpler.

http://www.428cobrajet.com/speedo-calc.html

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68 Coop
Gearhead

Posts: 4243
From: Mesquite, NV. 89027 M&M# 4256
Registered: Oct 2004

posted 05-15-2006 01:41 AM     Click Here to See the Profile for 68 Coop   Click Here to Email 68 Coop     Edit/Delete Message   Reply w/Quote
That one says I need an 8 with the 295 tires, so I'll just wait till I get my new tires and see what it says then. Thanks for the time and info.

------------------
William
The easiest way to find something lost, is to buy a replacement.

68 Coupe
289
C4
3:55's/Trac-Lok

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68mustang351w
Gearhead

Posts: 500
From: San Jose Ca
Registered: Sep 2004

posted 05-15-2006 01:42 AM     Click Here to See the Profile for 68mustang351w   Click Here to Email 68mustang351w     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by zieber:
I don't know for sure if this calculator works but it is simpler.

link


Just checked it and the calculator works but it assumes you have a 6 tooth drive gear. In order for it to be accurate, William (or anyone else with a 7 tooth drive gear would have to change both gears (drive & driven)... David F.

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68 Coop
Gearhead

Posts: 4243
From: Mesquite, NV. 89027 M&M# 4256
Registered: Oct 2004

posted 05-15-2006 08:06 AM     Click Here to See the Profile for 68 Coop   Click Here to Email 68 Coop     Edit/Delete Message   Reply w/Quote
Thanks David, I just want to get it right the first time, and do the easiest way possible.

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68 S-code GT
Gearhead

Posts: 3553
From: Sayreville, NJ, US
Registered: Mar 2000

posted 05-16-2006 08:06 AM     Click Here to See the Profile for 68 S-code GT   Click Here to Email 68 S-code GT     Edit/Delete Message   Reply w/Quote
This will help some with your RPMs.

http://www.ringpinion.com/content/calculators/RPM.asp

I know this has come up in the past (cause I asked about it once ) and there are a few formulas here somewhere but you have to find out what the outside diameter of your tires are.

------------------
Ed S.

68 S-code FB GT 4spd(now C6)/3.25 PS PDB
68 J-code(now 289) Cp Sprint"B" C4/3L00-9" PDB PS AC Bla-Bla-Bla
99 F150 XLT Ext/cab, 4X4, 5.4L, 3L55

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68 Coop
Gearhead

Posts: 4243
From: Mesquite, NV. 89027 M&M# 4256
Registered: Oct 2004

posted 05-16-2006 09:18 PM     Click Here to See the Profile for 68 Coop   Click Here to Email 68 Coop     Edit/Delete Message   Reply w/Quote
Thanks Ed.

I have 26.5 OD(P295/50R 15) on my back tires, and they are bigger than the front, so I take it I go with the biggest tire???

[This message has been edited by 68 Coop (edited 05-16-2006).]

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68mustang351w
Gearhead

Posts: 500
From: San Jose Ca
Registered: Sep 2004

posted 05-16-2006 09:27 PM     Click Here to See the Profile for 68mustang351w   Click Here to Email 68mustang351w     Edit/Delete Message   Reply w/Quote
You always go with the rears because those are the ones that are doing the pushing of the car... David F.

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68 Coop
Gearhead

Posts: 4243
From: Mesquite, NV. 89027 M&M# 4256
Registered: Oct 2004

posted 05-16-2006 09:47 PM     Click Here to See the Profile for 68 Coop   Click Here to Email 68 Coop     Edit/Delete Message   Reply w/Quote
Thanks David, just wanna be sure about this .

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68mustang351w
Gearhead

Posts: 500
From: San Jose Ca
Registered: Sep 2004

posted 05-17-2006 01:16 AM     Click Here to See the Profile for 68mustang351w   Click Here to Email 68mustang351w     Edit/Delete Message   Reply w/Quote
No problem, thats what we are here for.... David F.

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