Transmission to Speedometer Drive Gear

520SC · #1 (**permalink**) 03-14-2017, 07:47 PM

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?

CHANMADD · #2 (**permalink**) 03-14-2017, 08:23 PM

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.

twobjshelbys · #3 (**permalink**) 03-14-2017, 08:45 PM

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).

CHANMADD · #4 (**permalink**) 03-14-2017, 10:38 PM

That's right , the different colors will tell you which it is

jon@harrison.ne · #5 (**permalink**) 03-15-2017, 08:05 AM

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.

520SC · #6 (**permalink**) 03-15-2017, 09:36 PM

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.

Tom Wells · #7 (**permalink**) 03-16-2017, 04:01 AM

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

cobra 53 · #8 (**permalink**) 03-16-2017, 06:58 AM

Easy speed correction calculater.

Drivetrain / Speed correction

https://lmr.com

patrickt · #9 (**permalink**) 03-16-2017, 09:15 AM

Quote:

Originally Posted by Tom Wells

520,

It's on the speedometer cable end outside the trans and is easy to replace.

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

PeteH · #10 (**permalink**) 03-16-2017, 09:40 AM

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

Tom Wells · #11 (**permalink**) 03-16-2017, 06:19 PM

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

520SC · #12 (**permalink**) 03-16-2017, 06:29 PM

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.