Speedometer woes

[viven44]

On my 1978 Bronco, I cannot seem to figure out my speedometer troubles.

I have replaced the cable like 3 times (first time I blamed LMC.. second time I blamed the cruise control extras.. 3rd time I have to find something else to blame). Just can't keep it from jumping at 60+mph

The NP205 transfer case uses a plastic drive gear which is in somewhat bad shape. It is a 7 tooth drive gear, worn bad in the center. I cannot find a 7-tooth version (D3TZ17285A), but I can find one with 8 teeth (D5TZ17285A).

Ok I think I'm going to need some help also with the gear calculations. My speedo was supposedly correct per GPS app. Maybe it hasn't been. Also why I think I'm getting 7MPG

Drive gear teeth count = 7
Driven gear teeth count = 18 (is this correct?)
Rear end ratio = 3.5
Tire size = 31"

Now if I go to the 8 teeth version how does that change the calculations ?

[ArdWrknTrk]

It's interesting that all the charts show the 8R gear (D5TZ17285-A) for a 3.50/NP/C6 combo, regardless of tire circumference.



The charts are here with explanations:  https://www.garysgaragemahal.com/speedometer-gears-cables--sensors.html

The calculators:  https://www.garysgaragemahal.com/calculators.html

I'm not sure, are you asking what is 8/7?
Have you measured the actual radius, center to the ground?
Don't use diameter because inflation pressure and load  are going to distort that number.

Edit: the charts show either a 19T (pink) or 20T (black) 2A style driven gear depending on the tire.

[viven44]

Thanks Jim!!

I think it's all falling into place. I may have been wrong on many counts.

I think what I have is a 8R drive gear, and I did take out the old driven gear with 19 teeth and replaced it with one with 18 teeth. That is because I went from a 235/75/15 to a 31x10.5R15 tire size.

I guess I don't know how to count the teeth... here is a pic of the drive gear. It looks better in this pic but it is pretty bald in the center.

[viven44]

I will need to familiarize myself with the forum calculators for sure...

Per this calculator... I do have a working combination with the 31 inch tires... I just need to install the NOS 8R drive gear I just ordered and hoping that will stabilize everything.

https://www.ebay.com/itm/395341321592

https://www.tciauto.com/speedometer-gear-calculator

[ArdWrknTrk]

As I had to explain to Gary, you count teeth from the end, not across the gear...

[ArdWrknTrk]

I consider my 235/85 R-16's  M+S as a 32" tire....

I'm not sure how you come up with that #

[viven44]

Usually this is my reference
https://tiresize.com/comparison/

[ArdWrknTrk]

Your tire diameter is part of the size, but things will depend on remaining tread depth and again, how round your tires actually are.
I seriously doubt they are 10ply and running 75-80 psi "pretty close to round", regardless what counts is how far the centre of the axle is from the asphalt because this is going to determine rolling circumference. (the whole π r² thing..)

[viven44]

I intended to take a pic of the tire up close before leaving this morning. It is pretty darn round, especially in the back. Running 35PSI.

That sagging in my experience is also dependent on the wheel width vs tire width. Small wheel widths can cause the tire to sag more due to stresses on the sidewall... and I have seen that in the past.... so I agree.

[ArdWrknTrk]

Just get a measure from the center of the hub to the ground and use that number.

Definitely if you force too wide a tire on a rim, and it has some cheesy 2 ply passenger rating it's going to distort and overheat.
There's a reason for the difference between P rated & LT radials

[viven44]

Ok the speedo is happy now.

I had to lop off another 1/4 inch of the inner cable..... I had already cut a good 1/2 inch off on the driven gear side... There was still some built in stress I guess from it being too long so it was binding on me. It was bad... but I didn't register that it didn't register anything until about 10mph. Between 50-70mph, it was bouncing around so I was basically getting a root-mean-square input to the odometer  Maybe i'm slightly better than 7mpg after all

[viven44]

While the speedo was happy it wasn't perfect at high speeds like the one on Big Blue 2WD... I still had troubles over 60mph (live able but I wasn't happy). Just until this morning... when I went to work.

Swapped the LMC driven gear with OEM Ford driven gear CODD-17271-B today and drove 60 miles on the highway for an errand... I wanted the gear swap done before leaving so I got a good test run. Zero issues. Not even once did it want to jump or wander.... It is as good as Big Blue 2WD now... no doubts.

I say it was a case of LMC cheap driven gear that took me in circles and made me do so many things. Strike 3 against LMC... I got carried away into buying too much LMC junk for this Bronco build.

I can't even visually tell the difference to be honest. The OEM gear has R and 18 marked on it and the LMC one had no markings but obviously there are differences in tolerances of the teeth possibly. Both of them were the same cost as well  If only Ford still made / contracted all the parts for these old trucks no one would need to buy new vehicles!!

[BigBrother-84]

viven44 wrote

If only Ford still made / contracted all the parts for these old trucks no one would need to buy new vehicles!!

Touché.
How do you call this in English, a "truism"?

It's the sole and only thing that motivates their jobs.  Sale new vehicles.

[viven44]

I think Touché is probably the word in english as well in the urban circles, lol

In more local country circles... maybe "hell yeah" .. definitely not "bless your heart"

[BigBrother-84]

«Touché» means "right in the bullseye Man", so yep "hell yeah" does the job for sure!
You hit the nail on the head Vivek, Hell Yeah!

Yee-Haw!

[viven44]

Jim,

I owe you an apology.

I installed the NOS speedo drive gear today D5TZ-17285-A and I have about a 10-15% error now (Speedo shows 40, I'm actually going ~35) because the old gear was practically bald. The new one is driving the driven gear a bit more.

Any way you had mentioned the other day that I should measure the radius from center to ground and I had sort of not done that as I knew I was sure what I had was 31" tires and figured if I was off, it would be negligible as the tires looked round, but I am measuring only 14.25 from the ground so the effective diameter is only 28.5 in my case....

If I had paid closer attention to the chart you send, I would have noticed that depending on whether the application is highway or mud/snow/all-terrain, the highway variants always have a smaller driven gear as the effective tire diameter is likely larger.

I am off to changing my driven gear from 18 teeth to perhaps 19 or 20 teeth to get the speedometer more accurate based on the measured tire diameter.

[ArdWrknTrk]

I certainly don't need an apology, I was just surmising why your speedometer was off, and radius was the thing that made sense to me.  

Anyway, I hope changing the gear gets you a lot closer to ideal, and your mileage on track.  

[viven44]

After using the correct driven/drive gear combination, the speed calibration is good, especially at high speed. Now my gas mileage is 8.5mpg in city, up from around 7mpg. Not a far cry from the 11.5mpg my wife gets in the city with her brand new 'LS' with all the fancy DFM  and 10 speed trans

[ArdWrknTrk]

Your Edelbrock won't destroy the engine in 45,000 miles.  

[viven44]

Yes indeed!!

I spoke to the dealers about installing a DFM disable dongle, but apparently that can void the warranty. Well my wife was well advised of the perils of too much technology and my recommendation was to find a pre-2007 low mileage suburban but that wasn't reassuring from a "drive anywhere" standpoint to her.

She choose what to play with... for 3 years I don't have to worry about anything. For 5 years I don't have to worry about powertrain. After that I am on my own.. and not really sure how well I can service most things. Heck there is no transmission dipstick and changing transmission fluid requires removal of exhaust from both manifolds, at least there is an engine oil dipstick but that will be gone soon too I'm sure in newer models.