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Author Topic:   Speedometer Corrections
HHuggins
Gearhead

Posts: 182
From: Laguna Niguel, CA
Registered: Feb 2001

posted 11-29-2001 09:24 PM     Click Here to See the Profile for HHuggins   Click Here to Email HHuggins     Edit/Delete Message   Reply w/Quote
My 70 Mach I has an 8 in. rear instead of the standard 9. With the gearing it causes the speedometer to be off by about 12 mph. Is there anyway to correct this?

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_ _____________________ _

Harley D. Huggins II
Laguna Niguel, CA

1970 Mach I
1998 GT Convertible
www.geocities.com/hh_machi/

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HHuggins
Gearhead

Posts: 182
From: Laguna Niguel, CA
Registered: Feb 2001

posted 11-29-2001 09:44 PM     Click Here to See the Profile for HHuggins   Click Here to Email HHuggins     Edit/Delete Message   Reply w/Quote
I'll answer my own question. I did a search and it sounds like I need to find out the formula to figure out what gear I need in the speedo. Anyone know where to find it?

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_ _____________________ _

Harley D. Huggins II
Laguna Niguel, CA

1970 Mach I
1998 GT Convertible
www.geocities.com/hh_machi/

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DidgeyTrucker
Gearhead

Posts: 1060
From: Greenbrier, TN USA
Registered: Oct 99

posted 11-29-2001 11:34 PM     Click Here to See the Profile for DidgeyTrucker   Click Here to Email DidgeyTrucker     Edit/Delete Message   Reply w/Quote
At higway speeds, one tooth (on the driven gear - on the cable end) is worth about 5 mph.
If your speedometer reads fast, you need a gear with two more teeth. That's the gear that goes on the cable end that goes in the transmission.
If your speedometer reads slow, you need a gear with two less teeth.

Tracy

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Tracy M&M #245
1956 F-100 Panel w/429
(3.50 9")
1966 F-100 SWB w/351C
(3.25 9" - 3.70 posi for Saturday nights)
Music City F-100's,
1965 GT-350 S/C
(2.78 1st & 3.70 TracLok 9")
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Don't focus on the destination....make the JOURNEY the adventure
Visit my Web site: DidgeyTrucker's Website

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JT
Gearhead

Posts: 186
From: Pearland, Texas, USA
Registered: Oct 2000

posted 11-30-2001 09:23 AM     Click Here to See the Profile for JT     Edit/Delete Message   Reply w/Quote
Harley, you can change the speedometer gear. The formula to figure out which one you need is in the NPD catalog in the speedometer section.

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JT
'68 Coupe w/351W
'01 Bullitt #3357, Dark Highland Green

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SteveLaRiviere
Administrator

Posts: 33988
From: Saco, Maine
Registered: May 99

posted 11-30-2001 07:42 PM     Click Here to See the Profile for SteveLaRiviere   Click Here to Email SteveLaRiviere     Edit/Delete Message   Reply w/Quote
...and in our 'Articles' section:
https://mustangsandmore.com/ubb/speedogear.html

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'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
'97 Probe GTS 2.5L DOHC
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Doyle
Journeyman

Posts: 38
From: Annapolis, Maryland
Registered: Oct 2000

posted 12-02-2001 08:44 AM     Click Here to See the Profile for Doyle   Click Here to Email Doyle     Edit/Delete Message   Reply w/Quote

I am having a similiar problem with my
speedometer. I initially thought the
problem was in the trans but and was
going to have the trans guy check it out
this spring. Now you guys have me thinking
otherwise.

How can I check to see what rear my
70 Boss has?

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SteveLaRiviere
Administrator

Posts: 33988
From: Saco, Maine
Registered: May 99

posted 12-02-2001 06:09 PM     Click Here to See the Profile for SteveLaRiviere   Click Here to Email SteveLaRiviere     Edit/Delete Message   Reply w/Quote
Doyle, the most accurate way would be to put the rear on jack stands, mark the driveshaft and rear wheel, and count how many revolutions the driveshaft makes per wheel revolution. If the driveshaft turns 3.5 times per each rotation on the wheel, you have a 3.50 rear.

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'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
'97 Probe GTS 2.5L DOHC
My Photo Page
Christmas shopping? Why not use the Mustangsandmore.com Bookstore?

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Doyle
Journeyman

Posts: 38
From: Annapolis, Maryland
Registered: Oct 2000

posted 12-02-2001 07:25 PM     Click Here to See the Profile for Doyle   Click Here to Email Doyle     Edit/Delete Message   Reply w/Quote

Steve

Thanks - I pulled the history rpt from
ford and it says I have a 3.91 - now
to confirm.
What does the size of the rear end, as
in the original post 8 vs 9 in, have
to do with the ratio?

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SteveLaRiviere
Administrator

Posts: 33988
From: Saco, Maine
Registered: May 99

posted 12-02-2001 07:30 PM     Click Here to See the Profile for SteveLaRiviere   Click Here to Email SteveLaRiviere     Edit/Delete Message   Reply w/Quote
The 3.91 is your ratio. Your driveshaft should turn 3.91 times for every revolution of your wheels.

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'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
'97 Probe GTS 2.5L DOHC
My Photo Page
Christmas shopping? Why not use the Mustangsandmore.com Bookstore?

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