# Speedometer Driven Gear Suggestions



## SLSTEVE (Dec 8, 2020)

Getting close enough to start thinking about road worthiness. I have an M 20 with a 3.55 Sure Grip rearend and will be running 28 inch diameter rear tires. Which driven gear should I select? Chart in Restoration Guide says to use 19 tooth Natural gear for my application with all three of the tire sizes the factory installed back in the day. Anyone running this combination and using this speedo driven gear?


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## SLSTEVE (Dec 8, 2020)

SLSTEVE said:


> Getting close enough to start thinking about road worthiness. I have an M 20 with a 3.55 Sure Grip rearend and will be running 28 inch diameter rear tires. Which driven gear should I select? Chart in Restoration Guide says to use 19 tooth Natural gear for my application with all three of the tire sizes the factory installed back in the day. Anyone running this combination and using this speedo driven gear?


After a little more research I found that the 775 X 14 inch tires had a 27.02 inch diameter, and my BF Goodrich tires will be 28 inches. My question is: Will that .98 of an inch in the diameter differences be enough to throw my speedo off, if everything else is the same? 3.55 rear end, M20 transmission? How do you find the number of drive gears in the speedometer? THAT factor of the equation is the elusive part. Can’t seem to find any info pertaining to it.


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## 67ventwindow (Mar 3, 2020)

Did you try this ? Speedometer Driven Gears it might help


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## SLSTEVE (Dec 8, 2020)

67ventwindow said:


> Did you try this ? Speedometer Driven Gears it might help


Hi 67 vent,
Where do you find the drive gear number for an M20 Muncie? If I had that missing piece of the puzzle I could figure out which driven gear I need.


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## SLSTEVE (Dec 8, 2020)

SLSTEVE said:


> Hi 67 vent,
> Where do you find the drive gear number for an M20 Muncie? If I had that missing piece of the puzzle I could figure out which driven gear I need.


Guess it’s 8. Plugged that into equation and figure I need 20.45 so I’m going to order a 20 and a 21


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## 67ventwindow (Mar 3, 2020)

Thats good to hear. I have been trying to get this F O R D out of my garage but it keep biting me in the butt. I learned one thing from this car never take anything for granted. If it worked last time you looked at it it probably wont be working this time. If I could get this 326 to mount to the top loader I would be rid of that headache. That would be something go to a car show and have the wrong color blue motor under the hood.


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## SLSTEVE (Dec 8, 2020)

SLSTEVE said:


> Guess it’s 8. Plugged that into equation and figure I need 20.45 so I’m going to order a 20 and a 21


So I think I have the driven gear question figured out and I actually ordered a driven gear kit with multiple gears. When the kit arrives and I start out with a 20 tooth gear, I will hook up the speedometer cable and how does it route to the speedometer? There is NO information(that I could find anyway) in the Pontiac Restoration Guide. Can’t understand why there isn’t an Assembly Manual available for a 65 GTO. Thanks in advance, once again. You guys rock!


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## BearGFR (Aug 25, 2008)

Too late now I guess, but another approach would be to just put it together using what you already have, then go drive the car. Compare the speedo reading to the actual speed using a GPS app on your phone, then you can calculate how much correction you need and in which direction, then order the right gear. 
On my TH400, there are two different driven gear housings used. There's a cutover point in terms of driven gear tooth count that determines which housing is required. 

Bear


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## SLSTEVE (Dec 8, 2020)

BearGFR said:


> Too late now I guess, but another approach would be to just put it together using what you already have, then go drive the car. Compare the speedo reading to the actual speed using a GPS app on your phone, then you can calculate how much correction you need and in which direction, then order the right gear.
> On my TH400, there are two different driven gear housings used. There's a cutover point in terms of driven gear tooth count that determines which housing is required.
> 
> Bear


That’s just it, don’t have anything except the housing and the speedometer cable. I’m going to start with the 20 tooth gear and check from there. While we’re at it, just how does the speedometer cable get to the speedometer from the transmission?
Thanks again,
Steve


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## BearGFR (Aug 25, 2008)

On my 69, it goes forward under the car, through a clip that's near the pivot point for the trans "back drive" linkage (which doesn't exist on a 65), curls around, and enters through a hole in the firewall.


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## SLSTEVE (Dec 8, 2020)

BearGFR said:


> On my 69, it goes forward under the car, through a clip that's near the pivot point for the trans "back drive" linkage (which doesn't exist on a 65), curls around, and enters through a hole in the firewall.
> [/QUOTEI assume that it goes through the firewall on the driver’s side down low then?]


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