Speedometer Pinion

[Jonathan]

I am restoring a 1950 Plymouth Deluxe. I changed the rear axle to a '94 Ford Explorer with a 3.27 rear axle ratio. Now my speedometer is way off. I pulled speedometer pinion gear out the transmission, it is a 17 tooth (part # 652848).

 

How would I find how many teeth I need for the appropriate pinion gear and where would I find one? Thanks.

[keithb7]

I am not sure how to do the engineering math and figure out exactly what tooth count you require. However in the original parts books they listed all the optional speedo drives depending on how the car was configured.

 

What was your old rear end gearing? Divided by 17 on your old speedo pinion, maybe gets you a number. Your new rear end ratio 3.27,  divide it by your just calculated number.  Might give you a desired pinion tooth number so you have a similar overall ratio?

Edited April 26, 2022 by keithb7

[Jonathan]

Thanks for your reply.

 

I do not know what the original rear end gearing was. Possibly 4.10 or 3.73. I just bought the 1950 fastback and it had a seized engine and was dealing with lots of rust. So, I cannot see the printed / stamped gear ratio on the original rear axle. I still have the original rear axle, though.

 

As I do not have any printed material / original parts books, does anyone have access to the speed pinion options with the part numbers?  

[keithb7]

[keithb7]

[Jonathan]

Thanks, mine is the 3 speed transmission. Looks like a 3.27 gear ratio was not used in those days. Perhaps it was used on a similar (newer) vehicle?

[DonaldSmith]

You can get a ratio changer, which you add in line with the cable.  Police maintenance garages use them to correct the speed readings.   The professional kits are very expensive, because they include a full set of interchangeable gears.  But I found one on line a reasonable price.  Its ratio was 10 percent, but I don't remember whether it was up or down.   That was back in 2009. 

 

I mounted the ratio changer at the transmission, and modified the floor cover.  I should have added a short cable, to put the changer  more comfortably below the floor. 

 

[Sniper]

Two things affect the speedometer reading.  Gear ratio and tire size.  Since Chrysler never made a 3.27 ratio for that axle there won't be a pinion for it that works with teh stock tire size.  You could go with the 65246 pinion and a shorter tire to bring the speedometer back into line.  But a shorter tire also means high engine RPM at a given road speed, which negates the gear ratio change to an extent.

 

Since this isn't a restoration you might consider a GPS speedometer, if you are going 12v.  I used a new gauge panel to put aftermarket gauges in my 51, prices have gone way up I see.  Not sure if you can get the 50 version sans gauges, I was able to get the 51's without gauges.

 

https://boeseengineering.com/product-category/plymouth-gauge-panels/1949-1950-plymouth-car-gauge-panels/

 

 

[DJ194950]

Stewart-Warner sold one of those adaptors for years- not no more though.

 

See something like this showed up after just a minutes searching the web for "auto speedo gear ratio changer".https://transmissioncenter.net/shop/ratio-adapter-other-ratios-available-call-jim-1-888-2012066/

 

 

 

DJ

[Jonathan]

Wonderful, thanks very much everyone! 

[Loren]

Some good news for you.

You do not have to change to 12 volts to use a GPS or a phone charger for that matter.

The power adaptor has a circuit which regulates the variation in automotive charging systems to 5.5 volts.

That's a drop from 14.5 volts (12 volt system) or 7.5 volts (6 volt system).

The one thing you have to do is change the polarity.

I found out my speedo read 5 mph slow by using the GPS which was confirmed by the speedo repairman.

So while I had the speedometer out for repair I used the GPS as my speedo.

Easy peezy

[Sniper]

Not so easy to use if your Speedometer is of the GPS type, which is what I mentioned.  The GPS speedometer is installed in the dash and uses GPS signals to show your speed, there are no connections to the speedometer pinion.  Just power and ground, along with the included antenna.

 

It will NOT work on a USB source, 12v only.

 

An example https://www.autometer.com/3-3-8-speedo-140-mph-gps-ultra-lite.html

 

Or you can find a cellphone mount that works for you and use your cellphone as a speedometer.  That is definitely cheaper, lol.

Edited April 28, 2022 by Sniper

[Loren]

For clarity the GPS I am referring to is an old Garmin that I used before "Siri" on my iPhone (actually Google Maps).

I can't speak to a GPS speedometer as I've never used one.

Since my old (at least 5 years old) Garmin is somewhat obsolete (seems like everyone has an iPhone except for the luddites still carrying flip phones) I figured I didn't have much to loose if I burned it up. The maps are totally obsolete but the speedo works very well indeed.

[Sniper]

wow, I forgot about those things and I used to have one for work, lol.  I wonder where it ended up?