[grandam1979]

Got this on a trans I picked up over the weekend I was told it was removed from a mid seventies t/a it’s a turbo 350. I haven’t seen one before it’s marked 1.300 why would they have needed this ? For a low gear option car without transmission change?

[JLMounce]

These are utilized to correct speedometers when the drive/driven gear combination is not available for the rear end gear/tire combination.

I have one on my 69 bird for that reason.

[1969GiPper]

Originally Posted by grandam1979

Got this on a trans I picked up over the weekend I was told it was removed from a mid seventies t/a it’s a turbo 350. I haven’t seen one before it’s marked 1.300 why would they have needed this ? For a low gear option car without transmission change?

In 1989 I broke the 3:23 ring & pinion in my GP and replaced it with a 3:90. To get the speedo to read accurately I had to use a reducer. Ordered mine from the GMC Truck Center in Pontiac, Mi. Worked great. Removed it when I changed rear to a 3:36 gear a number of years ago. If I remember correctly there was a chart in the parts book to figure out what ratio and part number you needed.

[67drake]

I actually have on on my GTO. It came with 3.23’s, and when I went to 3.90’s I just used the reducer. It’s been on there so long, I can’t remember why I went that route. Maybe I couldn’t find the correct speedo gear. Sorry, getting old, and CRS.

[Ogre]

This worked for me many years ago. Performance and Transmission Center (PATC).
Good company to work with.

Ogre

[PDC]

Yes! I just got one of these about a month ago from this place:

https://transmissioncenter.net/shop/...1-888-2012066/

I had maxed out the number of teeth on the Driven Gear for my T56 and with a 26” tire and 3.70 rear gear, my speedo read exactly 10% fast. The 10% reduction is a ‘standard’ reduction that PATC had on the shelf and they shipped it right out. I installed it in under 10 minutes and the speedo reads dead on with my Phone app GPS speedo.

[carbking]

Have a similar device on my GTO. Because of the special 4-speed I used, speedometer cable from transmission not an option. Went with a electronic speedometer control that counted pulses from a device attached to the driveshaft, and then had a speedometer cable to the dash speedometer. Once programmed, it works as well as any gauge. Will work with any rear end ratio and any speedometer that accepts a cable.

Jon.

[Skip Fix]

My 78 TA with 2.56 rear gears had one.

[Verdoro 68]

I had one on my car for a while. I was running a late '70s Cadillac TH400 with a drive gear that had 18 teeth. They don't make a driven gear that would get my speedo to read correctly with 3.55s so I had to run a 17.6% drive reducer.

[Half-Inch Stud]

Have two GM Speedo cable 1:1 right angle units. No use

Have the Electric Pickup Speedo ASSY shown in Post 5. Neato

Speedo does work in my 68 4L80 combo anymore due to Drive Gear slipping on Tail shaft: must make amends.

Jon, got a link or name of youe ElecSpeedo Drive?

[Singleton]

Originally Posted by Skip Fix

My 78 TA with 2.56 rear gears had one.

My original drivetrain 78 T/A with 2.56 has one also