# speedometer gear size



## trimfixer (Sep 29, 2009)

I have a 65 GTO with the 2 speed auto trans. It has a 36 tooth speedo gear in it now and the speedo reads 70 mph when a gps is reading about 58 or 59 mph. Can anyone tell me what gear size I might need to get closer? Any ideas about how many mph each tooth change makes.


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## dimitri (Feb 13, 2009)

Usually a good rule of thumb is 3-4 mph per tooth. Good thing they are cheap and easy to replace. While you are in there I would replace the o-ring and little lip seal. I also like to put a dab of lithium grease on the end of the cable before putting it in.


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## trimfixer (Sep 29, 2009)

*Thank you*

Thank you Dimitri. That gives me a good idea where to start. Also thanks for the other suggestions. I will be sure to do all when I get the gear.
Steve


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## GTO JUDGE (May 14, 2005)

You can only go so far in replacing the spedo driven gear. The range of teeth with the maximum tooth gear may be stamped on the gears housing. A lot of getting this correct has to do with the correct gearing in the rear end. You can max out the gear and still be off if the gearing if the rear end is too high. If your rearend gearing was altered and the speedo gear was not, you'll need to know the gearing ratio so you can get the correct toothed gear. Just getting the highest tooth gear may not solve your problem. If you have oversized tires this too will add to a % over. 

Mine was off 20mph at 60mph. I had 4.10's. I installed the 2nd highest tooth gear I could install and it reduced it to 18mph over. I installed a new rearend with 3.55's and installed the highest toothed gear I could and I am spot on now up to 60mph and am off only 2mph. I am running 14x7 60 series tires which I attribute to the 2mph over. 

Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4


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## geeteeohguy (Feb 2, 2008)

I used to have a chart, but it's gone. You can probably find it on line...I'd bet on it. When I changed my 3.36 rear in the '67 to a 2.56, I changed from a 41 tooth gear to a 34-35 tooth gear. I also had to install an new transducer with the 34-35 tooth gear. For TH400's, there are two transducers: one for the tall gears and one for the shorter rear gears. With the 2 speed, I'm not sure. Mine ended up dead on. The gear was like $6, and the transducer (used) was $12.


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## Chazz (Mar 8, 2010)

Maybe this will help...

Speedometer gear charts


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## peahrens (Jun 7, 2010)

For my '66 I looked at this a few years ago and best I can remember I could not find a gear available that would get my 225-70/14 tires and 3:55 rear end to adjust the right % (vs a 1-mile speed test) from the gear currently installed. I concluded I'd have to go to an external speedo gear ratio box that a local guy can make up. In the end, I deferred a decision until next tire replacement...e.g., if I go to redlines the tire dimension will be different. It is annoying for the indicated speed to be 10mph high.


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## geeteeohguy (Feb 2, 2008)

peahrens, your 225/70/14 tires are about 26.4 inches tall...the same height as the original tires. I've run that same size on both my goats, have them on the '67 now, and all is good. My bet is that the gear in your trans is not matched to your rear gear ratio.


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## trimfixer (Sep 29, 2009)

*Thanks*

Thank you all for the great replys to my problem. Looks like I have a lot more work to do to get it corrected.


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