Speedo Pinion
- Thread starter Omni
- Start date
EDIT: I had this wrong. Changed to correct.
Larger (more teeth) to spin slower, more RPM
Smaller (less teeth) to spin faster, less RPM.
I had it figured out once what the change per tooth was once... But that was a while ago and I can't remember now.
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Thanks. I have been meaning to look up the gear I need to correct my speedo. My Imperial originally came with tall L84/15 and a 3.23 gear, with the lower 235/75R15 I have a speedo approximately 6/7 mph too fast at 60 mph. The service manual shows 29 teeth for an L84/15(tall tire) and a 29 tooth for a J78/15 (235/75R15)
Time to start searching the bins. Are the pinions colour coded for amount of teeth?
Went through my stash. I found 2-27 tooth. 2-32 tooth and 5-29 tooth gears. I will give the 32 a try, maybe get back some of the extra miles the fast speedo piled on over the years.
Time to start searching the bins. Are the pinions colour coded for amount of teeth?
Went through my stash. I found 2-27 tooth. 2-32 tooth and 5-29 tooth gears. I will give the 32 a try, maybe get back some of the extra miles the fast speedo piled on over the years.
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Just Carbs
Well-Known Member
Are you talking about the gear on the end of the cable?
Less teeth spins the cable faster and shows more mph? More teeth slows it down?
Yes, look in the FSM
Geez... Now you have me questioning my answer.
Looking at the chart, a larger tire would need a slower RPM gear. Smaller tire would need a faster RPM gear.
Does that make sense?
Just Carbs
Well-Known Member
Chart says a bigger tire spins the driveshaft slower so fewer teeth are used to speed the cable up.
Just Carbs
Well-Known Member
But, the real problem is, what do we do when the cable spins, but the needle don't move?
Thank you all for the responses.
Yes, I am referring to the gear at the end of the cable.
All the charts I have found are for '66 on up when the speedo gears were re-engineered. I am working on a '65 with the small speedo gear (16tooth currently). My tire size is 28"diameter (225/75/15), with a rear gear of 2:76. Original tire (8.25-14) is, from what I researched, 26". Haven't found any chart that deal specifically with bias ply, so, using what they offer as metric equivalents is where I got the 26" from.
Yes, I am referring to the gear at the end of the cable.
All the charts I have found are for '66 on up when the speedo gears were re-engineered. I am working on a '65 with the small speedo gear (16tooth currently). My tire size is 28"diameter (225/75/15), with a rear gear of 2:76. Original tire (8.25-14) is, from what I researched, 26". Haven't found any chart that deal specifically with bias ply, so, using what they offer as metric equivalents is where I got the 26" from.
Then we actually have to crawl under the dash... and we don't want to do that anymore.....
Brewer's Performance - Mopar A833 4-Speed Transmission and Component Specialists
I can at least get that right this evening.
Just Carbs
Well-Known Member
Wrong answer, we go GPS on the windshield.
3175375
Senior Member
I went through this with my 65 Mustang. Here’s what I did:
I bought an assortment of the gears and smartly played musical gears, until I had the closest reading higher than true. I used gps and mileage markers for truth data for the speedometer and odometer.
D Cluley
Active Member
The FSM says your Newport should have come with 8.25-14 tires, 2.76 rear end & a 17 tooth pinion.
Based on a couple of different charts/calculations an 8.25-14 tire should have been about 27.25" tall. Putting that into the formula on the Brewster's page, the perfect gear for 8.25-14 & 2.76 is 16.3 teeth. Their chart recomends a 16 tooth, so Chrysler rounded up, Brewster's rounds down.
Your current 225/75-15 should be about 28.25" which splits the difference between 8.55-14 & 9.00-14 which were the 2 larger tires found on 300s & New Yorkers. According to the FSM both of them would use a 16 tooth pinon if they had a 2.76 rear. The Brewster's chart recomends a 16 as well, and their formula comes out to 15.8 teeth.
So, my conclusion is that your car should have had a 17 from the factory, so if you think it has a 16 it has been changed, but that is what you want for the current tires anyway.
My real question is, how accurate is your odometer? The speedometer itself can need calibration and that will throw things off, but the odometer is a straight gear drive, so that will tell you if the pinion is a problem. If it ever stops raining, go out for a nice long drive. Use GPS to go exactly 50 or 100 miles and see what the odometer says. That makes it easy to calculate the percentage of error.
Based on a couple of different charts/calculations an 8.25-14 tire should have been about 27.25" tall. Putting that into the formula on the Brewster's page, the perfect gear for 8.25-14 & 2.76 is 16.3 teeth. Their chart recomends a 16 tooth, so Chrysler rounded up, Brewster's rounds down.
Your current 225/75-15 should be about 28.25" which splits the difference between 8.55-14 & 9.00-14 which were the 2 larger tires found on 300s & New Yorkers. According to the FSM both of them would use a 16 tooth pinon if they had a 2.76 rear. The Brewster's chart recomends a 16 as well, and their formula comes out to 15.8 teeth.
So, my conclusion is that your car should have had a 17 from the factory, so if you think it has a 16 it has been changed, but that is what you want for the current tires anyway.
My real question is, how accurate is your odometer? The speedometer itself can need calibration and that will throw things off, but the odometer is a straight gear drive, so that will tell you if the pinion is a problem. If it ever stops raining, go out for a nice long drive. Use GPS to go exactly 50 or 100 miles and see what the odometer says. That makes it easy to calculate the percentage of error.
Dan
Thanks for the info. The Brewers information is now 'booked marked'.
I could not find much, if anything. on dimensional data for the 8.25-14 tire.
I currently have a 16 tooth speedo gear so this looks like it is going to be a non-issue.
Regarding odometer accuracy, using the interstate mile markers as a guide, the odo currently is shy about 500 feet. Of course that is a guesstimate on my part. I do not have the 15" on the car yet so that accuracy may change.
Thanks Again
Omni
Thanks for the info. The Brewers information is now 'booked marked'.
I could not find much, if anything. on dimensional data for the 8.25-14 tire.
I currently have a 16 tooth speedo gear so this looks like it is going to be a non-issue.
Regarding odometer accuracy, using the interstate mile markers as a guide, the odo currently is shy about 500 feet. Of course that is a guesstimate on my part. I do not have the 15" on the car yet so that accuracy may change.
Thanks Again
Omni
How can you have pinion gears with different numbers of teeth, presumably different tooth spacing and yet they will still mesh correctly with what-ever gear is driving them?
FWIW, they do make ratio adaptors too. Chrysler used one on cars with steep gears and small tires. It had a .8:1 ratio. You can see this listed in a '65 Plymouth FSM. You can get them in various ratios. I don't think they still make them, but I could be wrong.
Here's one I just grabbed at random on eBay. Stewart Warner Speedometer Gear Adjuster Adapter - 666 F 1.1111 Ratio | eBay
No colours in the Service manual, the colours show up in parts manual.
Looking at them right now in the '65 FSM, but yes, I would expect the colors would be listed in the parts manual too.
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