Speedometer gear

[chryslerwagon]

I am replacing the differential gear set on my series V. The current ratio is 4.22 and I'm installing a 3.89. It has a 14 tooth speedometer drive gear on the tail shaft and I'm not sure if I will need to change it in order to get a correct reading. If it needs to be changed what should the new one be?

 

[hartmandm]

8% reduction ...

Was the speedo accurate with the 4.22 ratio? People often change tire sizes versus the original tire sizes and introduce some error +/- that way. So good to know what percentage difference you really need to address.

I don't know if other drive gears are available. Another option is to add a gear reducer.

Mike

 

[chryslerwagon]

Speedo was about 2 to 3 MPH slow at 60. It's not the original. I built it out of three spares. I'm not sure I can get it accurate enough with a change in ratio of about 8%. I calculate it will now be about 10 MPH slow. Looking to see if anyone else has already solved the problem.

 

[Mike O'D]

I don't think there are different gears available - the speedometer is what changed with different ratios.

 

[volvoguys]

The factory parts manual makes no mention of a different speedo for different gearing. Now, if the car was fitted with an overdrive box the speedo was different.

Series V were fitted with 4:22 rears until body B395010313, when changed to 3:89. But parts manual 6600992 shows no different speedo, even back to Series I, regardless of rear gearing.

M ..... v

 

[Barry]

I am replacing the differential gear set on my series V. The current ratio is 4.22 and I'm installing a 3.89. It has a 14 tooth speedometer drive gear on the tail shaft and I'm not sure if I will need to change it in order to get a correct reading. If it needs to be changed what should the new one be?

In 4th gear (1.00:1) at any given engine RPM, changing the rear axle gear from 4.22:1 to 3.89:1 will cause the actual vehicle speed to increase by 8.4% without affecting the speedometer reading. If "a 14 tooth speedometer drive gear on the tail shaft" refers to the gear on the transmission end of the speedometer cable, then changing from a 14 tooth gear to a 13 tooth gear should increase the speedometer reading by about 7.7%. The number of teeth on the speedometer cable must be a whole number, so 13 teeth is as good as it gets.

I have no idea where you could source a suitable 13 tooth gear or if the S-V speedometer cable-end gear can be readily changed. In the 1960's, it was common for U.S. car makers to use easily interchangeable speedometer cable-end gears, but I have never heard of Rootes doing that.

As Mike noted in post #2, there is a gizmo called a speedometer calibration gear box that can be custom made to correct just about any error.

https://speedometercablesusa.com/gear_box_adapters.html

https://www.ihpartsamerica.com/store/SPEEDO-GEARBOX.html

In theory, you could re-calibrate your current "built ... out of three spares" speedometer by reducing the spring tension on the dial side of the "speed cup". No idea how practical that might be.

 

[JSLoeffl]

Parts Manual 6600992 Jan 68 5th Issue Page GA5 Remarks state: Non-overdrive models Check type when ordering.
item A109 Speedometer Driving Pinion (13 teeth) Non-overdrive Part # 1207586
item A109 Speedometer Driving Pinion (14 teeth) Non overdrive Part # 1207587
Rick at Sunbeam Specialties should still have them. Last spring I ordered the 14T one from him.
My 1967 Alpine V B395010782 has a 3.89 rear end, is Non-overdrive and had as original a 13T gear in the transmission. Speedo was close to accurate with old B78x13 tires which were about 24" diameter.
With the non-availability of 13" tire choices, I now have 185/70R13 tires which are 23.2" diameter. With these tires my speedo reads a little fast (at 63 mph on speedo, actual GPS mph was about 60)(odometer gain was about .06 per mile from actual). Note: I also recently had my speedo rebuilt by Nisonger so speedo calibration should not be an issue.
I replaced the 13T pinion gear with the 14T pinion gear (this will turn the speedo cable slower).
My speedo is now very accurate at 60 mph. At lower speeds on the way up to 60 mph, the speedo reads slow. Let's say at about 25 mph on the speedo, you may actually be going 30, but as your speed increases, the speedo gets closer to actual.
My guess is the parts manual lists the 2 pinion gears available depending on if you have the 4.222 differential or the 3.889 differential since the speedometer itself did not change as noted by volvoguys. Some Series V non-overdrive vehicles came with either differential depending on country it was marketed in or serial number according to the parts manual. I assume the 13T w/3.889 and the 14T w/4.222
I have elected to keep the 14T installed so that my speedo is fairly accurate at highway speeds. If I later get larger diameter tires, I will put the 13T back in.
chryslerwagon, I assume you are talking about a non-overdrive vehicle w/4.222. That may be why you have a 14T pinion gear. Changing to the 13T gear may be the option to get you closer with your differential change, but tire sizing will play a significant role. Changing from 4.222 to the 3.889 would be a similar change as installing larger diameter tires.

 

[hartmandm]

Nice find in the Parts Manual. Screen snippet below.



The OD gearboxes changed out the pinion gear from series 3 on:



Sunbeam Specialties catalog:

 

[chryslerwagon]

Parts Manual 6600992 Jan 68 5th Issue Page GA5 Remarks state: Non-overdrive models Check type when ordering.
item A109 Speedometer Driving Pinion (13 teeth) Non-overdrive Part # 1207586
item A109 Speedometer Driving Pinion (14 teeth) Non overdrive Part # 1207587
Rick at Sunbeam Specialties should still have them. Last spring I ordered the 14T one from him.
My 1967 Alpine V B395010782 has a 3.89 rear end, is Non-overdrive and had as original a 13T gear in the transmission. Speedo was close to accurate with old B78x13 tires which were about 24" diameter.
With the non-availability of 13" tire choices, I now have 185/70R13 tires which are 23.2" diameter. With these tires my speedo reads a little fast (at 63 mph on speedo, actual GPS mph was about 60)(odometer gain was about .06 per mile from actual). Note: I also recently had my speedo rebuilt by Nisonger so speedo calibration should not be an issue.
I replaced the 13T pinion gear with the 14T pinion gear (this will turn the speedo cable slower).
My speedo is now very accurate at 60 mph. At lower speeds on the way up to 60 mph, the speedo reads slow. Let's say at about 25 mph on the speedo, you may actually be going 30, but as your speed increases, the speedo gets closer to actual.
My guess is the parts manual lists the 2 pinion gears available depending on if you have the 4.222 differential or the 3.889 differential since the speedometer itself did not change as noted by volvoguys. Some Series V non-overdrive vehicles came with either differential depending on country it was marketed in or serial number according to the parts manual. I assume the 13T w/3.889 and the 14T w/4.222
I have elected to keep the 14T installed so that my speedo is fairly accurate at highway speeds. If I later get larger diameter tires, I will put the 13T back in.
chryslerwagon, I assume you are talking about a non-overdrive vehicle w/4.222. That may be why you have a 14T pinion gear. Changing to the 13T gear may be the option to get you closer with your differential change, but tire sizing will play a significant role. Changing from 4.222 to the 3.889 would be a similar change as installing larger diameter tires.

Thank you for your answer. It looks like you spent a lot of time on it and I really appreciate it. I will probably be getting a 13 tooth gear from Rick or Mark but I'll install the 3.89 diff and do a road test to find how far off it is first. BTW my ser# is 39501476 and non-overdrive with the 4.22

 

[chryslerwagon]

In 4th gear (1.00:1) at any given engine RPM, changing the rear axle gear from 4.22:1 to 3.89:1 will cause the actual vehicle speed to increase by 8.4% without affecting the speedometer reading. If "a 14 tooth speedometer drive gear on the tail shaft" refers to the gear on the transmission end of the speedometer cable, then changing from a 14 tooth gear to a 13 tooth gear should increase the speedometer reading by about 7.7%. The number of teeth on the speedometer cable must be a whole number, so 13 teeth is as good as it gets.

I have no idea where you could source a suitable 13 tooth gear or if the S-V speedometer cable-end gear can be readily changed. In the 1960's, it was common for U.S. car makers to use easily interchangeable speedometer cable-end gears, but I have never heard of Rootes doing that.

As Mike noted in post #2, there is a gizmo called a speedometer calibration gear box that can be custom made to correct just about any error.

https://speedometercablesusa.com/gear_box_adapters.html

https://www.ihpartsamerica.com/store/SPEEDO-GEARBOX.html

In theory, you could re-calibrate your current "built ... out of three spares" speedometer by reducing the spring tension on the dial side of the "speed cup". No idea how practical that might be.

I'm able to calibrate it to any one speed by using the clock spring but it will only be accurate at that one speed. I would like it to be correct at most driving speeds and have the odometer be correct also. I know my way around the insides of these having spent three years working at a speedometer shop. I just need to begin with correct parts to get optimum results. Calibration boxes can be a nightmare to get right and can be more expensive than doing it right.

 

[Tim R]

If I were you I would test drive the car once you have made your changes and note the speed you are showing compared to the speed that a Sat Nav app shows before you do anything. One of the speedo reconditioning guys once told me that there are around 30 different internal gears available for Smiths/Jaegar speedos and that if you accurately measure the number of revolutions of the speedo cable over a set measured distance (typically 100ft) they can choose the gears to ensure that it reads correctly regardless of the diff, tyre size etc that you are running. I have a 3.7 diff on my car and the speedo is very accurate but that was more by luck than judgement! At 30mph, it shows 30. At 40 it shows about 41-42 and stays in the similar range up to a genuine 70mph where the speedo is showing about 76.

Tim R