viven44 Posted July 30 Share Posted July 30 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 ? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted July 30 Share Posted July 30 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. Link to comment Share on other sites More sharing options...
viven44 Posted July 30 Author Share Posted July 30 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. 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. Link to comment Share on other sites More sharing options...
viven44 Posted July 30 Author Share Posted July 30 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. 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 Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted July 30 Share Posted July 30 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. As I had to explain to Gary, you count teeth from the end, not across the gear... Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted July 30 Share Posted July 30 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. I consider my 235/85 R-16's M+S as a 32" tire.... I'm not sure how you come up with that # Link to comment Share on other sites More sharing options...
viven44 Posted July 30 Author Share Posted July 30 I consider my 235/85 R-16's M+S as a 32" tire.... I'm not sure how you come up with that # Usually this is my reference https://tiresize.com/comparison/ Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted July 30 Share Posted July 30 Usually this is my reference https://tiresize.com/comparison/ 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..) Link to comment Share on other sites More sharing options...
viven44 Posted July 30 Author Share Posted July 30 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..) 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. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted July 30 Share Posted July 30 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. 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 Link to comment Share on other sites More sharing options...
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