Speedometer Gear Charts - Clarification/Differences

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69cudaownr

69cudaownr

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I recently changed my speedometer gear due to a rear end change. I found the following information out while deciding what speedometer gear I needed.
Almost all sites point to an axle ratio to tire size chart that lists the corresponding teeth count such as below;
speedochart by tire measurement.jpg

I queried tiresize.com and looked up my 215/70/R14 tires which are 25.9". So, in my case the 3:23 axle ratio with a 26" tire requires a 33 tooth gear according to the above chart.

BUT, not convinced I looked in my 1969 Service Manual and on Page 21-96 found a speedometer pinion gear chart as shown below;

Speedometer Gear Chart.jpg


I then queried Vintage Tire Size Conversion Chart. This chart provides different tire sizes used during different eras, ie. pre 1964, 65-72, etc. I found that my 215/70/R14 tires were equal to a 7.75x14 or an F70x14. If you look at the service manual chart above you will find that this tire with a 3:23 requires a 32 tooth speedo gear. Not a 33 tooth as the first chart suggested. So I chose the 32 tooth gear and have found that my speedometer is exact with my GPS.
Many of you may have to convert your 16, 17 or 18 inch tires to a usable 70 series 14 inch or 15 inch tire in order to use the Vintage Tire Size Conversion Chart to correspond with the Service Manual Chart. This can be done by going to the tiresize.com website and using their comparison chart, OR their tire size chart. Enter a tire size and you will see tabs to choose comparable 16", 17", 18" etc rims. You may have to do the comparison more than once, meaning enter your 18" tire find the 16" comparable, enter the 16" to find the 14" or 15" comparable. Then use that tire size to find the Vintage Tire Size Conversion and find your speedo gear in the Service Manual Chart.
It's a little work but may answer why you've been thinking your speedo is out a bit.
I hope this helps someone in the future.
 
69cudaownr said:
I recently changed my speedometer gear due to a rear end change. I found the following information out while deciding what speedometer gear I needed.
Almost all sites point to an axle ratio to tire size chart that lists the corresponding teeth count such as below;
View attachment 1715532669
I queried tiresize.com and looked up my 215/70/R14 tires which are 25.9". So, in my case the 3:23 axle ratio with a 26" tire requires a 33 tooth gear according to the above chart.

BUT, not convinced I looked in my 1969 Service Manual and on Page 21-96 found a speedometer pinion gear chart as shown below;

View attachment 1715532679

I then queried Vintage Tire Size Conversion Chart. This chart provides different tire sizes used during different eras, ie. pre 1964, 65-72, etc. I found that my 215/70/R14 tires were equal to a 7.75x14 or an F70x14. If you look at the service manual chart above you will find that this tire with a 3:23 requires a 32 tooth speedo gear. Not a 33 tooth as the first chart suggested. So I chose the 32 tooth gear and have found that my speedometer is exact with my GPS.
Many of you may have to convert your 16, 17 or 18 inch tires to a usable 70 series 14 inch or 15 inch tire in order to use the Vintage Tire Size Conversion Chart to correspond with the Service Manual Chart. This can be done by going to the tiresize.com website and using their comparison chart, OR their tire size chart. Enter a tire size and you will see tabs to choose comparable 16", 17", 18" etc rims. You may have to do the comparison more than once, meaning enter your 18" tire find the 16" comparable, enter the 16" to find the 14" or 15" comparable. Then use that tire size to find the Vintage Tire Size Conversion and find your speedo gear in the Service Manual Chart.
It's a little work but may answer why you've been thinking your speedo is out a bit.
I hope this helps someone in the future.
Click to expand...
Someone posted this a few months ago............

chart.jpg
 
I like the factory chart. There will always be a trade off, either too fast or too slow.

My personal favorite is the know your current tooth count, and get a GPS and speedo value, and apply that percentage to the current tooth count, you can decide if you want to round up or down.
 
A56 said:
Someone posted this a few months ago............

View attachment 1715532702
Click to expand...
That chart is identical to the first one I posted. And I bought my speedo gear from Brewers but did not use their guide. In my case their chart was wrong. 26” tire with 3:23 ratio is a 32 tooth, not 33. Confirmed by the Plymouth Service Manual.
 
Useful.
I'm glad I searched first before I asked.
Thank you
 
Just a note...

Someone recently noted that their speedo was spot on for speed but off for miles.

Meaning, clocking mile markers, 5 miles was 4.9

But gps and speedometer read 60 mph.

Turns out there is an adjustment inside the speedo head. First get the gear the correct size for miles, then adjust the adjuster for the correct speed.

A bit hard to do and you can not do it on the car.
 
69cudaownr said:
I recently changed my speedometer gear due to a rear end change. I found the following information out while deciding what speedometer gear I needed.
Almost all sites point to an axle ratio to tire size chart that lists the corresponding teeth count such as below;
View attachment 1715532669
I queried tiresize.com and looked up my 215/70/R14 tires which are 25.9". So, in my case the 3:23 axle ratio with a 26" tire requires a 33 tooth gear according to the above chart.

BUT, not convinced I looked in my 1969 Service Manual and on Page 21-96 found a speedometer pinion gear chart as shown below;

View attachment 1715532679

I then queried Vintage Tire Size Conversion Chart. This chart provides different tire sizes used during different eras, ie. pre 1964, 65-72, etc. I found that my 215/70/R14 tires were equal to a 7.75x14 or an F70x14. If you look at the service manual chart above you will find that this tire with a 3:23 requires a 32 tooth speedo gear. Not a 33 tooth as the first chart suggested. So I chose the 32 tooth gear and have found that my speedometer is exact with my GPS.
Many of you may have to convert your 16, 17 or 18 inch tires to a usable 70 series 14 inch or 15 inch tire in order to use the Vintage Tire Size Conversion Chart to correspond with the Service Manual Chart. This can be done by going to the tiresize.com website and using their comparison chart, OR their tire size chart. Enter a tire size and you will see tabs to choose comparable 16", 17", 18" etc rims. You may have to do the comparison more than once, meaning enter your 18" tire find the 16" comparable, enter the 16" to find the 14" or 15" comparable. Then use that tire size to find the Vintage Tire Size Conversion and find your speedo gear in the Service Manual Chart.
It's a little work but may answer why you've been thinking your speedo is out a bit.
I hope this helps someone in the future.
Click to expand...
Do these charts work with manual transmissions also?
 
69cudaownr said:
I recently changed my speedometer gear due to a rear end change. I found the following information out while deciding what speedometer gear I needed.
Almost all sites point to an axle ratio to tire size chart that lists the corresponding teeth count such as below;
View attachment 1715532669
I queried tiresize.com and looked up my 215/70/R14 tires which are 25.9". So, in my case the 3:23 axle ratio with a 26" tire requires a 33 tooth gear according to the above chart.

BUT, not convinced I looked in my 1969 Service Manual and on Page 21-96 found a speedometer pinion gear chart as shown below;

View attachment 1715532679

I then queried Vintage Tire Size Conversion Chart. This chart provides different tire sizes used during different eras, ie. pre 1964, 65-72, etc. I found that my 215/70/R14 tires were equal to a 7.75x14 or an F70x14. If you look at the service manual chart above you will find that this tire with a 3:23 requires a 32 tooth speedo gear. Not a 33 tooth as the first chart suggested. So I chose the 32 tooth gear and have found that my speedometer is exact with my GPS.
Many of you may have to convert your 16, 17 or 18 inch tires to a usable 70 series 14 inch or 15 inch tire in order to use the Vintage Tire Size Conversion Chart to correspond with the Service Manual Chart. This can be done by going to the tiresize.com website and using their comparison chart, OR their tire size chart. Enter a tire size and you will see tabs to choose comparable 16", 17", 18" etc rims. You may have to do the comparison more than once, meaning enter your 18" tire find the 16" comparable, enter the 16" to find the 14" or 15" comparable. Then use that tire size to find the Vintage Tire Size Conversion and find your speedo gear in the Service Manual Chart.
It's a little work but may answer why you've been thinking your speedo is out a bit.
I hope this helps someone in the future.
Click to expand...
Makes sense to me. I have a 3.23 with a 225/60R14, I chose a 33 tooth gear because I wanted my speedo to read a bit slow. But it turned out that the speedometer is bang on the GPS, and the odometer is off by 0.6% (I have a speedometer app that measures distance travelled. I compared actual distance according to the app to the odometer on 4 different trips of about 40 miles each. All trips were virtually identically off the odometer reading by 0.6%).

In case anyone is wondering, (I'm sure you're not) I wanted the speedo to read slow because I'm running a factory Magnum EFI setup with PCM controlling the 46rh shift to OD and lockup. I wanted to trick the PCM into thinking I'm going slower than I actually am so it shifts a bit later. I went from a 31 to a 33 so it's better, just not the performance orientation I was looking for. And since I'm in Canada the road signs don't match the speedo anyway, so the perfect reading is only marginally helping me.
 
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