Speedometer Gear Accuracy problems-I'm stumped

[BriceRoad]

Some years back (could be as much as 12 years ago) I installed 3.91 gears in my car. Many times in the past I have used the charts in places like Brewer's (just for example) to determine the number of teeth of the speedometer gear and I was always able to get the speedometer accurate to within 1 MPH. My car has P25560/R15 tires which are 27.05" in diameter. The chart said use a 39 tooth gear (at least I thought it did-that could have been a mistake). I installed it properly with the little line of the 39-45 pointing towards 6:00. I could have sworn when I first did this it was accurate but now I am not too sure since it was years ago.

Recently I noticed that the speedometer seemed inaccurate. I checked it with the app "Waze" and indeed it is off by about 10%. When it says 77 MPH I am actually doing 70MPH (give or take 1MPH). So this time instead of using a chart I used several speedometer gear calculators that I found on the internet. One thing I needed to know was how many teeth are on the output shaft of the transmission. According to Brewer's all '66 and up transmissions (manual or auto) have 13 teeth on the output shaft. I don't know what year my transmission is but I highly doubt it is '65 or older. I have a Passon 4 speed OD so I am assuming it also has the 13 teeth. I know for a fact it was accurate when I had the 3.23 gears. I checked it with my Garmin GPS back then. Anyway, each calculator said I need a 38 tooth gear. So I bought a 38 tooth gear and installed with the 33-38 line pointing towards 6:00. The initial problem to me was, would not I have had to use a larger number of teeth to make the speedometer show a lower number of MPH? Although it didn't make sense I went ahead and finished installing the 38 tooth gear. Now here is the strange part. Nothning changed! It's still off by 10%!! It still reads 10% too high. Something should have changed. First thing I did was I double checked the teeth on both gears. Yup, the first one has 39 teeth and the second one has 38 teeth. Then I tried to count the teeth on the output shaft and it only seems like 10 or 11 teeth but it is very difficult to see since I don't have a lift. I could easily be wrong about that. What am I doing wrong? Is it possible the speedometer went bad and no matter what gear I install it will always be off by 10%? I am totally stumped on this.

 

[Katfish]

Measure mileage, to check odometer.
Good chance the speedo is the problem.
They are magnetically coupled, sometimes the bushings wear out and it's not accurate.
Happened to me.

 

[BriceRoad]

Thank you very much. I will check that out.

 

[Steve340]

The chart is most likely for a 4 speed which is 1 to 1 in top gear.
You say you have OD.
This would alter the output shaft RPM.

 

[Steve340]

I used your figures and used the Summit calculator and it said use a 37 tooth.
But you are right to slow down the speedo reading based on your driving data you would in theory use a larger tooth count.
I would just try a 40 or 41 tooth and see what happens.

 

[pschlosser]

guessing and trial and error worked for Edison.

 

[B5blueGC]

I tried the online calculator and my speedo was off. I went back to the service manual and used their chart. Bought the suggested gear and now I am about 1 MPH off at all speeds.

 

[Ricks72Chlgr440]

I tried the online calculator and my speedo was off. I went back to the service manual and used their chart. Bought the suggested gear and now I am about 1 MPH off at all speeds.

Put a little more air in your tire or take a little out and you will be right on, joking!

 

[mrmopar340]

At least if you drive by the current speedometer you'll never get a ticket cause you'll never be speeding.

 

[BriceRoad]

Thanks everyone for all the replys.

Steve340: you're that the RPMs will be different with overdrive but the number of teeth on the output shaft will not change. After thinking about it and if all '66 and up transmissions have 13 teeth than I highly doubt Jamie Passon would build a transmission with a different number of teeth from that because then everyone who bought his OD trans would have to use a different way of figuring out which speedometer gear to use. I will confirm this when I see him in Ohio. As I mentioned I know it was accurate when I had my 3.23 rear gears and I also had the OD trans at that time. That is strange. I don't why the Summit calculator would be different.

B5BlueGC: Very cool suggestion! I never knew the FSM had one of those charts. I pulled out my 1970 FSM and checked it out. It only has the old tire sizes like G70x15 for example so I had to go online and I found a chart to convert the old tire sizes to modern tire sizes. Tire Rack
The size that is equal to my P255/60R15 is 7.60x15. The closest size to that in the factory chart is 7.75x15 and then when I follow it over to 3.91 gears and it also calls for a 38 tooth gear.

I'm leaning to it being the speedometer. I have a spare that I will change to. This will also give me a chance to install the electronic voltage regulator I bought from R/T Engineering. I was talking to the owner of "The Gauge Doc" at Carlisle and he recommends these electronic regulators because if the instrument cluster voltage regulator should pass a lot of voltage to the gauges due to some failure of the main electronic voltage regulator than that would be catastrophic for the gauges. The R/T engineering until will shut down in an over voltage/current situation thus saving the gauges. Realistically, if you have the standard gauges like me they are cheap enough to replace but for anyone with the Rallye gauges I would definitely get the R/T unit. I just think it's cheap insurance for $70.

 

[BriceRoad]

Well, I am back to being stumped. I changed the speedometer and no change. It's still off about 2-3 mph in the 20s and 30s then goes to about 4-5 mph off in the 40s. I couldn't go much faster in my neighborhood. I started thinking maybe I got different tires than I thought I had because I did get new tires about 1.5 years ago. Nope they are P255/60R15s like I thought and I even tried measuring them. I got just under 27" but that isn't that accurate. It was close enough to know they are 27" tall. I guess I will try a 40 tooth gear next but i am going to try that with the second speedometer which is unknown to me. I just picked it up at a swap meet to use a spare along with the gauges. I think it is highly unlikely that both speedometers would be bad but I guess it could happen. I do have a 3rd speedometer I could try. At least I got the voltage regulator installed for the gauges.

 

[B5blueGC]

I have never used one, but I looked online at their web page. They show 2 transmissions for OD, an 18-spline and a 23-spline. Have you called them and asked if they have any knowledge on this?

 

[BriceRoad]

I sent an email over to Passon Performance asking about the teeth on the output shaft of the OD transmission. Jamie's OD transmission with the OD gears he designed only comes in 18 spline. I heard him say this to a prospective buyer at Carlisle. The one I have is 18 spline. There was a factory Chrysler 23 spline OD transmission but it is not designed to hold up to high HP.

I decided to ask AI about this problem. According to AI (which we all know isn't always correct) it could be the speedometer. AI told me some possibilities about the speedometer that I didn't know:

• Weakened Hairspring: The speedometer needle is held back by a tiny hairspring. Over 30 to 50 years, this metal spring loses its tension (fatigues). Without enough resistance to counter the spinning magnet, the needle swings too far forward, reading consistently fast.
• Degraded Internal Dampening Oil: Mechanical gauges use a thick silicone dampening fluid on the needle pivot to keep it steady. If this fluid dries up or migrates, the needle will overshoot its actual speed.
• Magnet Over-Magnetization: While rare, the spinning drive magnet can sometimes collect magnetic debris or change properties, pulling the speed cup harder than it should.

 

[Steve340]

I think I would just try the 40/41 tooth driven gear - it is easy to do at least.