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Speedometer RPM to Speed ratio? #2836942
10/24/20 01:46 PM
10/24/20 01:46 PM
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Oregon
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AndyF Offline OP
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Anyone know what the speed to mph ratio for a speedometer is? For example, 1000 rpm on the cable equals what MPH? I played with it a little bit and it seems like the ratio is around 16:1 but there has to be specification somewhere for it. If the ratio is 16:1 then 1600 rpm on the cable would equal 100 mph on the speedometer.

Re: Speedometer RPM to Speed ratio? [Re: AndyF] #2836951
10/24/20 02:03 PM
10/24/20 02:03 PM
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Rio Linda, CA
John_Kunkel Offline
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1000 rpm = 60 mph. 1000 turns = one mile on the odometer.


I'm supposed to respect my elders, but its getting harder and harder for me to find one now.
Re: Speedometer RPM to Speed ratio? [Re: John_Kunkel] #2836964
10/24/20 02:32 PM
10/24/20 02:32 PM
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Oregon
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AndyF Offline OP
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Okay that makes sense. I figured it was some sort of standard ratio. My guess wasn't too far off, looks like the ratio is 16.7:1

I wonder if all speedometers are the same? I'm working on a conversion that uses some GM parts and it appears that the speedo ratio that GM used is the same as the Mopar one. The speedo gear chart for a GM transmission is the same ratio as Mopar chart. The numbers are different since the number of teeth on the shaft are different but the ratios are the same. The Richmond transmission I'm using has a 8 tooth gear on the main shaft while a 727 transmission has a 13 tooth gear, but the ratio between the driveshaft speed and the speedometer cable speed is the same for both. Which leads me to believe that GM also used the 1000 RPM cable speed for 60 mph.

Re: Speedometer RPM to Speed ratio? [Re: John_Kunkel] #2836992
10/24/20 03:59 PM
10/24/20 03:59 PM
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Oregon
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AndyF Offline OP
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I think I figured it out. My rear tires are 88 inches circumference so 60 mph is 720 revs/mile. With a 3.23 rear gear that makes the driveshaft speed 2326 rpm. To get to 1000 shaft speed for the speedo cable I need a 2.326:1 ratio gear reduction. The Richmond has a 8 tooth gear on the main shaft so I need 18.60 teeth on the speedo gear. GM offers both a 18 tooth and a 19 tooth gear so I'll start with the 19 tooth gear and see how it does. It should read 2% slow if my math is correct. I don't know how well the speedometer itself is calibrated. It is the original speedometer in my '65 Coronet so it might be a little wore out.

Re: Speedometer RPM to Speed ratio? [Re: AndyF] #2837022
10/24/20 05:21 PM
10/24/20 05:21 PM
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West Plains, MO
DrCharles Offline
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Remember that the tire diameter * pi is not exactly the same as the rolling diameter. The tire flattens on the bottom due to the load and contact patch...

For example, my 295/50R15 tires (according to the manufacturer) have a diameter of 26.61", or a circumference of 83.60". That would mean that a mile (63360") requires 757.9 revolutions. But the specified revolutions per mile is 3% larger at 781.

So the 19 tooth gear is most likely to be the correct one.

Re: Speedometer RPM to Speed ratio? [Re: AndyF] #2837271
10/25/20 12:40 PM
10/25/20 12:40 PM
Joined: Feb 2005
Posts: 680
St Louis, MO 63026
convx4 Offline
mopar
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St Louis, MO 63026
Your original 1965 speedometer will read slow and the mileage should be accurate. Inside the speedometer is a magnet that becomes week over time allowing a slow reading on the needle. A speedometer repair place over charges the still magnet. Then with test equipment that has been calibrated to 60 cycles in the U.S. or 50 cycles Europe is then tun to 1000 RPM. The speedometer will read faster than 60 MPH. With a magnet charger the spinning magnet is zapped until the needle reads 60 MPH. The bushing in the head of the speedometer becomes worn over time, causing all kinds of issues.

Re: Speedometer RPM to Speed ratio? [Re: DrCharles] #2837317
10/25/20 02:05 PM
10/25/20 02:05 PM
Joined: Jan 2003
Posts: 21,651
Rio Linda, CA
John_Kunkel Offline
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Rio Linda, CA
Originally Posted by DrCharles
Remember that the tire diameter * pi is not exactly the same as the rolling diameter.


This is probably the most important part of speedo calibration, advertised diameter varies. My most recent experience was with a tire that had an advertised diameter of 28" but was actually 26 3/4" computed from the rollout.


I'm supposed to respect my elders, but its getting harder and harder for me to find one now.
Re: Speedometer RPM to Speed ratio? [Re: John_Kunkel] #2837386
10/25/20 06:23 PM
10/25/20 06:23 PM
Joined: Dec 2004
Posts: 9,920
Rittman Ohio
fourgearsavoy Offline
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Rittman Ohio
You can get a more accurate measurement from a road force tire balancer as it applies force to the tire while it measures the actual diameter. I've measured slicks before on our balancer at the shop and it works very well twocents

Gus beer

Last edited by fourgearsavoy; 10/25/20 06:23 PM.

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