Speedometer Gear with Dana 20 HD 32 spline rear output

[abrogate932]

I rebuilt my 302, 3 speed and Dana 20 over the last year and recently got everything back in my 1971 Bronco. During the process, I decided I was finally going to address my speedometer gear being slightly off by swapping the 18 tooth drive gear to the supposed correct 16 tooth. I am running 31" Toyo Open County A/T III tires and stock 3.55 gears. Before the tear down of the powertrain, I would show approximately 9 MPH faster than I was actually going, reading 70 MPH on the dash when GPS shows me at 61 MPH.

During the tear down of the Dana 20, the lock nut on the rear output ripped all the threads off the end of the shaft and left me needing to replace the output due to the damage. At the time I could not locate an OEM replacement and ended up getting the 32 spline replacement from Toms Offroad. The install went smooth and the problem was solved. I swapped the 16 tooth drive gear onto the cable and when I was finally able to take the truck for a test drive, I was now showing 80 MPH on the dash and 62 MPH on GPS. After talking with Toms about the scenario, we verified that the speedo gear on the 32 spline shaft is 7 tooth and the 16 tooth drive gear with 31's and 3.55's matches the charts.

I have read in other speedo gear threads that the rolling radius of the tire is a better measurement to go off of, not the actual tire diameter (30.7" according to Toyo). I don't have the rolling radius measurement on me at the moment.

Researching the solution I came across this thread which lays out the math to determine how to choose the drive gear.

https://classicbroncos.com/forums/threads/which-speedo-gear.170533/post-1660997

If I was to follow Broncobowsher's post #3:

18 tooth gear has a 9 MPH error/61 MPH actual = 14.75% error x 18 tooth = 2.66 Teeth. Round up to 3 teeth + 18 = 21 Tooth Gear.
16 tooth gear has a 18 MPH error/62 MPH actual = 29% error x 16 tooth = 4.65 Teeth. Round up to 5 teeth + 16 = 21 Tooth Gear.

So that leads me to believe the 21 tooth gear would correct the speedometer reading on the dash, right? Am I completely butchering the math? The more teeth on the gear would result in a slower speedometer reading which is what I need.

But what I can't make sense of is all the charts showing a 16 tooth gear where the math shows a 21 tooth gear. Can anyone share some insight here? Could the 32 spline output shaft diameter be a contributor to this situation?

 

[Yeller]

Couple of things

First it is entirely possible that your speedometer was recalibrated sometime in the past for different tires, gear ratio, ect. So the math doesn’t lie.

Second rolling radius is a myth unless you are running bias ply tires. I know there is going to be push back but physics and construction material properties are involved. With modern steel belted radial tires the circumference stays the same regardless of rolling diameter, the steel belts dictate that, and just like a track on a bull dozer the distance traveled is the same for every revolution of the tire. There are some variables with items like rubber density that do allow that to change but those influences are small in the overall scheme.

So yes your math is sound

 

[abrogate932]

Couple of things

First it is entirely possible that your speedometer was recalibrated sometime in the past for different tires, gear ratio, ect. So the math doesn’t lie.

Second rolling radius is a myth unless you are running bias ply tires. I know there is going to be push back but physics and construction material properties are involved. With modern steel belted radial tires the circumference stays the same regardless of rolling diameter, the steel belts dictate that, and just like a track on a bull dozer the distance traveled is the same for every revolution of the tire. There are some variables with items like rubber density that do allow that to change but those influences are small in the overall scheme.

So yes your math is sound

I was thinking along similar lines in regard to the rolling radius being irrelevant due to the circumference not changing.

Thanks for the insight.

 

[abrogate932]

Couple of things

First it is entirely possible that your speedometer was recalibrated sometime in the past for different tires, gear ratio, ect. So the math doesn’t lie.

Second rolling radius is a myth unless you are running bias ply tires. I know there is going to be push back but physics and construction material properties are involved. With modern steel belted radial tires the circumference stays the same regardless of rolling diameter, the steel belts dictate that, and just like a track on a bull dozer the distance traveled is the same for every revolution of the tire. There are some variables with items like rubber density that do allow that to change but those influences are small in the overall scheme.

So yes your math is sound

I am now curious about the possibility of the speedometer being recalibrated previously. My initial thought would be that the calibration is done at the transfer case output with the swapping of speedo drive gear, 16t through 22t. I realize there are 6 and 7 tooth gear options on the shaft itself. What would a speedometer shop do in the speedometer? I am guessing there is a corresponding drive on the speedometer end that they could fool with? We used to have a speedometer shop locally, but it’s been gone for almost two decades now.

 

[Yeller]

I am now curious about the possibility of the speedometer being recalibrated previously. My initial thought would be that the calibration is done at the transfer case output with the swapping of speedo drive gear, 16t through 22t. I realize there are 6 and 7 tooth gear options on the shaft itself. What would a speedometer shop do in the speedometer? I am guessing there is a corresponding drive on the speedometer end that they could fool with? We used to have a speedometer shop locally, but it’s been gone for almost two decades now.

It’s a voodoo science as far as I’m concerned but do know a guy that does calibrate them, but he won’t do Ford stuff I tried, he says he has plenty to do.

 

[DirtDonk]

Before the tear down of the powertrain, I would show approximately 9 MPH faster than I was actually going, reading 70 MPH on the dash when GPS shows me at 61 MPH.

I swapped the 16 tooth drive gear onto the cable and when I was finally able to take the truck for a test drive, I was now showing 80 MPH on the dash and 62 MPH on GPS.

Well, charts or not, you went the wrong direction no matter what.
A smaller gear on the cable (driven gear) will make the speedometer read faster. A larger gear will slow it down.

Looks like the 21 will get you closer than you've been.

Paul

 

[abrogate932]

I finally got the correct speedo gear. I bought a 21 and a 20 tooth gear. The 21 tooth was close, but I was reading slightly slower than my GPS speedo app. I swapped to a 20 tooth gear and finally the dash matches the GPS. The 31" Toyo Open County A/T III tires measure 30" exactly with 30 psi. Referring back to the charts, with the 3.55 gears I should be running a 16 tooth, but the 20 tooth did it. I am still curious what the variable is that is different about my setup. It was suggested that the speedometer itself could have been fooled with along the past 50 years and now I am leaning that way. Hopefully this info is helpful to someone in the future.

 

[Yeller]

Glad you got it sorted out.

On a related note, dealing with the same speedometer drive parts (atlas). I changed tires of the same "size" and the speedometer now reads 2-3mph slow, rather than 1-2mph slow. Proves that everything is affected by everything else, and it all has to play nice together. I'm assuming about the time the tires are worn out it will be spot on again.

 

[DirtDonk]

Do you have a circumference specification/measurement on both the old and new tires by any chance? How old were the old tires?
Probably the old specifications have been changed at least once by the manufacturer during the ensuing years, so current info might not be consistent. But maybe there's some old info available. Especially if they were not that old to begin with.
Just curious what might have changed in that regard. I had almost the same change when changing from 35" General Grabber M/T's to BFG M/T's back in the '90's.
At the time, there was an approx. 1/2" change in overall diameter (as measured on the truck) but I did not think to compare circumference measurements back then. And the results matched my expectations regarding radius/diameter anyway.
This was long before I'd seen a chart, or bothered with the math. Just noting differences in general.

Paul

 

[serial car restorer]

I (as a former tire sales/service guy) never use circumference or radius. Dig through the manufacturer's website to find the revs per mile instead. Comparing revs per mile is the most accurate way to estimate changes from size or manufacturers.

 

[DirtDonk]

That’s what I was thinking. Isn’t that the actual specification they put in the stats anyway? Rather than circumference?

 

[serial car restorer]

Generally, yes. Over the years when I did that kinda thing for a living (spec'ing custom wheel and tire fitments for hot rods and such) I found that the RPM figures were typically very accurate, to the point that using a little math I could predict the speedo error virtually every time.

 

[Yeller]

Rpm wasn’t listed for either tire so it’s all a guess