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03-14-2017, 08:47 PM
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CC Member
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Join Date: Feb 2017
Location: Ewa Beach,
HI
Cobra Make, Engine: 1966 Street Beasts Cobra 427 S/C, 502 cid
Posts: 121
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Not Ranked
Transmission to Speedometer Drive Gear
I have a C6 3-speed transmission. With a 3.73 rear end gear setup, my speedometer reads fast, about 27% too fast. Most speedometer driven gear calculators require you to input the number of teeth on the drive gear. Is there a way to access the speedometer drive gear so that I can count the number of teeth as I turn the rear wheels?
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Todd
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03-14-2017, 09:23 PM
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CC Member
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Join Date: Mar 2006
Location: San Marcos california,
CA
Cobra Make, Engine: 1989 KCC from South Africa Right Hand Drive
Posts: 1,601
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Pull the speedo drive out... oil is going to come out... Then put a paint mark on one tooth ...Oh , I meant to tell you to first jack it up..at least the rear wheels off the ground...so you can turn the driveshaft by hand and count the teeth.
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03-14-2017, 09:45 PM
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CC Member
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Join Date: Jan 2009
Location: Las Vegas,
NV
Cobra Make, Engine: Shelby CSX4005LA, Roush 427IR
Posts: 5,584
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There are transmission/rear end/tire size calculators out there. I had to get a pink gear (as I recall). Took 5 minutes to change it out (fortunately I had a friend that had a lift so access was easy).
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Cheers,
Tony
CSX4005LA
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03-14-2017, 11:38 PM
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CC Member
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Join Date: Mar 2006
Location: San Marcos california,
CA
Cobra Make, Engine: 1989 KCC from South Africa Right Hand Drive
Posts: 1,601
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That's right , the different colors will tell you which it is
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03-15-2017, 09:05 AM
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CC Member
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Join Date: May 2007
Cobra Make, Engine: JCF supplied the body,frame,and some of the driveline-I completed it to be a very accurate 289 slabside
Posts: 149
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Seems to me that you could just pull out the speedo gear, count the teeth, and multiplty that number by 1.27 and get the gear with the nearest number of teeth.
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03-15-2017, 10:36 PM
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CC Member
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Join Date: Feb 2017
Location: Ewa Beach,
HI
Cobra Make, Engine: 1966 Street Beasts Cobra 427 S/C, 502 cid
Posts: 121
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Thanks for the replies, guys.
After some research, I've found that on a C6, the drive gear can only be accessed by removing the transmission tailpiece. So, it appears that jon@harrison.ne's method is the way to go. I just loaded SpeedView (an Android GPS-based speedometer) on my cell phone so I can more accurately determine the percentage difference between my speedometer and actual vehicle speed.
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Todd
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03-16-2017, 05:01 AM
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Senior Club Cobra Member
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Join Date: Mar 2000
Location: St. Augustine,
FL
Cobra Make, Engine: E-M / Power Performance / 521 stroker / Holley HP EFI
Posts: 1,929
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520,
This page may help you: TCI Speedometer Gears and Accessories | JEGS
The driven gear is most likely the one you need to replace, and there's a selection shown near the top along with ways to estimate which one may work for you.
It's on the speedometer cable end outside the trans and is easy to replace. Some fluid may try to escape so take some care...
Count the teeth on the existing gear, then select a new one that is closest to the number of teeth you decide you need.
Near the bottom there's a clip you may need.
Tom
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Wells's law of engine size: If it matters what gear you're in, the engine's too small!
Last edited by Tom Wells; 03-16-2017 at 05:03 AM..
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03-16-2017, 07:58 AM
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CC Member
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Join Date: May 2006
Location: Denia / SPAIN,
ESP
Cobra Make, Engine: NAF 289 Blue/White strips
Posts: 188
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Easy speed correction calculater.
Drivetrain / Speed correction
https://lmr.com
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Rico, NAF 289
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03-16-2017, 10:15 AM
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Half-Ass Member
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Join Date: Jun 2005
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 22,005
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Quote:
Originally Posted by Tom Wells
520,
It's on the speedometer cable end outside the trans and is easy to replace.
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On ERA cars, it's a medium PITA to get the hold down bolt back in. Here's a pic of mine after I had applied some Torque Seal to it.
if you're going to pull your speedo cable off, you might has well lubricate it if you've seen even a hint of needle bounce. A detailed thread on that can be found here: Lubricating the Smiths Speedometer Cable To Stop Needle Bounce
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03-16-2017, 10:40 AM
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CC Member
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Join Date: Mar 2017
Location: Vermilion,,
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Cobra Make, Engine: FF 289Comp #8955
Posts: 42
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I don't know that you'll be able to see in the hole where the speedo cable goes in to count the drive gear while in the car. Even out of the car it would be a bit tricky.
I have a C4 mated to a diff with 3:70 gears in a Triumph TR6. KAR mustang parts in Columbus, Ohio had the correct speedo gear, reads spot on much to my surprise. The car is offsite in storage presently, otherwise I could probably find the part # in the book that is in the trunk. I seem to remember different speedo gears are different colors, but it's been 10 years since I built that one, and cannot remember. Gearing is too high numerically, shifts almost immediately and revs are up around 2800 at 60mph. I'd like to change to something like 3:50 gear, but choices are limited with the TR6 rear diff.
PeteH
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03-16-2017, 07:19 PM
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Senior Club Cobra Member
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Join Date: Mar 2000
Location: St. Augustine,
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Cobra Make, Engine: E-M / Power Performance / 521 stroker / Holley HP EFI
Posts: 1,929
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Pete,
Check my post again - the driven plastic gear is the one to replace - not the drive gear.
The driven gear is the easy one to count and change since it is installed with a little clip onto the end of the speedometer cable - you just remove the bolt, pull the cable & gear out and do the swap.
Patrick,
You'll note I didn't say it was necessarily easy to get back in
Tom
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Wells's law of engine size: If it matters what gear you're in, the engine's too small!
Last edited by Tom Wells; 03-16-2017 at 07:21 PM..
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03-16-2017, 07:29 PM
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CC Member
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Join Date: Feb 2017
Location: Ewa Beach,
HI
Cobra Make, Engine: 1966 Street Beasts Cobra 427 S/C, 502 cid
Posts: 121
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The easiest calculation that can be done without knowing the number of teeth on the drive gear is this. Needed number of driven gear teeth = Current number of driven gear teeth x (car speed as indicated by the speedometer / car speed as indicated by a GPS). The faster you're going when you measure the speeds indicated on the speedo and the GPS, the more accurate the result will be.
How much oil can I expect to leak out of the C6 when I pull the sending unit out? Being that I'll have to do it twice (once to count the number of existing teeth, and again to install the new driven gear) that could end up being a very messy job. And Redline ATF isn't exactly cheap stuff.
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Todd
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