Nylon Speedometer Gear

[Model A Ron]

Well I got my Speedometer working with a new cable but now it's reading 5 to 8 MPH off with my GPS depending on speed. I see Mike's sells Nylon Speedometer Gears however they offer 17 to 21 tooth gears. Anyone know what gear to get?

Thanks
Ron

[McMimmcs]

Originally Posted by Model A Ron

Well I got my Speedometer working with a new cable but now it's reading 5 to 8 MPH off with my GPS depending on speed. I see Mike's sells Nylon Speedometer Gears however they offer 17 to 21 tooth gears. Anyone know what gear to get?

Thanks
Ron

Ron it may well not need a new gear, possibly just an adjustment. Wayne

[Model A Ron]

Originally Posted by McMimmcs

Ron it may well not need a new gear, possibly just an adjustment. Wayne

Wayne
May I ask how do you adjust the speedometer? Is this in the Red book as I have that in my shop.

Thanks
Ron

[Model A Ron]

Also I just took the bottom section off and the nylon gear does not have any apparent wear on it.

[gary678*]

5-8 MPH difference might beyond the adjustment. Is it 5-8MPH low or high ? I had the same issue which was 5% low. I changed from a 19 tooth gear to an 18 tooth gear ( one revolution is quicker) and it is now spot on. If it too fast go one or two gears higher (one revolution is slower). I think you will solve the problem.

[Model A Ron]

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Mine is reading 5 to 8 MPH slow depending on how fast I go. The faster I go the larger the gap from GPS to the cars speedometer.

[McMimmcs]

My solution has on two occasions been to send it to Papanik. Hopefully he will comment on your post.

[Bob C]

Is the odometer off also??

[Model A Ron]

Originally Posted by Bob C

Is the odometer off also??

I do not know at this point.

[gary678*]

In my post #% I should have mentioned that both my speed and odometer were 5% off: the gear change resolved both issues.

[gary678*]

Typo mistake in #10... should read "post # 5".

[GRutter]

There are other factors in determining which speedometer gear you need.
Are you running the standard 19 or 21" tires for your vehicle? Is your differential ratio the standard 3.78? If standard, you would likely be using an 18 tooth gear.

Check your odometer - If it is correct, then you should have the correct gear and need your speedometer calibrated. If it is off for the distance you have driven, then your gear may be incorrect.

[Model A Ron]

I have 21'' stock wheels and a 3.78 ratio......would that be a 18 tooth gear as well?

[GRutter]

There were only 2 speedometer gear and cap assemblies originally supplied for 3.78 ratio cars, and both were 18 tooth.

Part A-17270-DR (Cap & Gear Assy) was used with the oval speedometer from 1928-30 for use with the square drive speedometer cable, and part A17270-F was used with the round speedometer 1930-32 for use with the keyed drive speedometer cable.

The gear used in both of the above assemblies was A-17271-B (18 tooth) and was driven by the 7 tooth Speedometer Driving Gear Part A-17285.

There is obviously a degree of inaccuracy in using the same 18 tooth gear for both 19" and 21" tires, and I believe this is why many believe that a different gear would or should be used.

Given a standard 3.78 differential, in 3rd Gear at 2000 RPM:
- with 19" tires the vehicle would be travelling 47.0 MPH
- with 21" tires the vehicle would be travelling 48.5 MPH

Regardless, the speedometer should read 46.7 MPH.
So given 2000 RPM * 7 Driving Gear / 18 Driven Gear = 777.8 Speedometer Cable Rotational Speed.

Speedometers are calibrated so that 1000 RPM = 60 MPH, resulting in the speedometer indicating 777.8 RPM x 60 MPH /1000 RPM = 46.7 MPH... probably "close enough".