Speedometer Pinion Gears for optional tire sizes and final drive gear ratios 1949-56

[Loren]

I have an admittedly tight focus on the KT Keller cars, so that's what I research and play with. It's all about having fun after all and those are the ones I like.

I am a huge fan of Overdrive transmissions because they don't really change the charm of the cars and actually add to it.

When you put one in your car your speedo will be off at all speeds because the speedometer pinion gear was selected for a 4.1 ring and pinion.

The good news is you can swap out the pinion from your old three speed to the O.D. and all will be back to normal ( assuming your speedometer doesn't need service ).

But what if you changed the rear end ratio or the tire size?

I don't advocate changing rear axles because I don't see much to be gained there. You can easily change the differential carrier ( aka 3rd member ) in a Plymouth if you want another ratio. Where I live there's lots of open road with highway speeds of 55 to 70 so optional ratios, tires and O.D. go well with my Plymouth.

My P17 Coupe came with 6.40-15 tires ( modern equivalent 195/75/15 ) and a 3.73 final drive ratio so the speedometer pinion would be a 17 tooth gear ( p/n 652-848 ) for an example.

Chrysler designed the pinions to use the same 6 tooth gear on the output shaft so you only need to change one gear to correct the speedometer.

One thing you find out right away is selecting speedo gears is not an exact science. You can get close and you might hit it right on the button but you more likely will be over or under by 2 or 3 mph. That's why the Highway Patrol has an enforcement tolerance.

 

So let's look at tires first

6.40 15 = 195/75/15 standard tire for a P17

6.70 15 = 205/75/15 standard tire for a P18

7.60 15 = 225/75/15 an optional size tire

 

Final drive ratios

3.90 very common on long wheelbase cars

3.73 standard ratio on many optional on all

3.54 optional

 

Chart of ratios & tires vs speedometer pinions

3.90  6.70 15  17 tooth pinion

3.73  6.40 15  17 tooth pinion

3.73  6.70 15  16 tooth pinion

3.54  6.70 15  16 tooth pinion

All      7.60 15  16 tooth pinion

 

So you can see you're not going to get an exact reading but you can get close enough to avoid a ticket. The part number for the 16 tooth pinion is 652-846. I have found that if you have the number for an obscure part the parts house will look to find it in their inventory but they are likely to blow you off if you just say "I need a 16 tooth speedometer gear."

Happy hunting!

 

[martybose]

While your description is good, in the case of an R10 OD it is wrong.  The R10 moves the speedometer pickup to the tailshaft of the OD unit, so it is reading the propshaft speed, which stays the same regardless of the engagement of the OD function.  That said, it i probably easier to get an inline speedometer gearbox to correct any error that exists with the final drive ratio/tire size issue.  Most speedometer shops can supply them with any gear ratio that you need, just tell them how the speedometer reading and a GPS speed compare.

 

Marty

[FarmerJon]

For the sake of consolidating information, here is what I have found regarding the speedometer pinions. 

These were cropped from pictures posted by @keithb7

My '55 OD trans had a 18T nylon gear in it. It was sealed with a replaceable lip seal.

The mid '47 trans I stripped for parts had a steel 17T gear, with the rubber seal moulded into the housing. 

The rubber seal would be difficult to reproduce. The housing could be machined to take the same lip seal as the later one. 

I found that Timken 311189 was a good fit as a replacement. 

 

I will add the dimensions for machining the housing, as soon as I find where I wrote them down....

Hopefully this adds to the good information that @Loren took the time to post.

[Loren]

5 hours ago, martybose said:

While your description is good, in the case of an R10 OD it is wrong.  The R10 moves the speedometer pickup to the tailshaft of the OD unit, so it is reading the propshaft speed, which stays the same regardless of the engagement of the OD function.  That said, it i probably easier to get an inline speedometer gearbox to correct any error that exists with the final drive ratio/tire size issue.  Most speedometer shops can supply them with any gear ratio that you need, just tell them how the speedometer reading and a GPS speed compare.

 

Marty

 

My point was that O.D. cars had final drive ratios in the neighborhood of 4.10 ( some places say 4.30 ) and the speedometer pinion that went with it. Thus no matter what gear you were in the speedometer would read incorrectly. Which is true even if that line was unnecessary ( my bad ). All transmissions that have speedometer gears measure the driveshaft rotation speed. Some exceptions are for example VW Beatles measure left front wheel speed and SAAB 99/900 which have the gear on the differential. You can buy speedometers that measure GPS signals and the are dead on with no connection to the wheels or driveshaft but they are really pricey.

What I was trying to get across was that you can correct your speedometer reading with an easy swap of the speedometer pinion gear. If you want an O.D. transmission ( R10 G1 ) you can change the speedo pinion with the one in your old transmission. However if you swap differential carriers to get the ratio you want, you need to score the speedo pinion from that car. There is an exception here in that the 3.54 ratio is usually in an automatic transmission car and that speedometer pinion is not the same design as the manual transmission.

Slight changes in tire size can get you closer as well.

[Young Ed]

Pro tip. If you have a bunch of transmissions and go hunting through them for the proper gear to match the 3.73 you swapped into your coupe make sure you reinstall all the wrong ones correctly. Otherwise later on when Dad goes to install one of those in place of the OD he's not selling with the 50 Plymouth bad things will happen to the new owner. Like the tail housing exploding....

[DJK]

For the speedo pinion seal, I found a Honda #91204-HB3-004, is a perfect fit.

[Kilgore47]

You can also down load a GPS speedo app on your smart phone.  That's what I used on the P15 until I got the real speedo fixed.  Then I used the phone app to check the accuracy of the real speedo.  We used to use the mile markers on the highway to check the accuracy of the car speedometer and odometer.  GPS is much faster now.