Author Topic: correct speedometer gear to get  (Read 1466 times)

Offline truetriplex

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correct speedometer gear to get
« on: April 21, 2021, 10:41:04 AM »
I have done the formula to order the correct speedometer gear for my car, but when I installed it, my speedometer says that I am doing 60, but my GPS says that I am doing 52. How may teeth different do I need to go to correct the situation?

The car has a 350 rear axle, and I am running 235 ? 60 x 15? tires/wheels. If i did the formula correct, I should have a 19 tooth gear.  Based on this information, what gear (tooth count or color) should I have to get a correct reading?  transmission is a C6 with a 7 tooth drive gear in the transmission.

Offline RoyceP

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Re: correct speedometer gear to get
« Reply #1 on: April 21, 2021, 10:53:02 AM »
More teeth on the driven gear = speedometer reads faster.


So in your case it sounds like one less tooth on the driven gear would be better / more accurate.
1968 W code 427 Cougar XR-7 GTE Feb 23 Dearborn C6 / 3.50 open
1968 R code 428CJ Cougar XR-7 May 13 Dearborn C6 / 3.91 T - Lock

Offline 67gta289

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Re: correct speedometer gear to get
« Reply #2 on: April 21, 2021, 11:42:32 AM »
MPC info

D0OZ-17271-A = purple with 21 teeth.  The 20 tooth gear is orange C8SZ-17271-B
« Last Edit: April 21, 2021, 11:47:02 AM by 67gta289 »
John
67 289 GTA Dec 20 1966 San Jose
7R02C156xxx
MCA 74660

Offline jwc66k

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Re: correct speedometer gear to get
« Reply #3 on: April 21, 2021, 05:44:55 PM »
I have done the formula to order the correct speedometer gear for my car, but when I installed it, my speedometer says that I am doing 60, but my GPS says that I am doing 52. How may teeth different do I need to go to correct the situation?

The car has a 350 rear axle, and I am running 235 ? 60 x 15? tires/wheels. If i did the formula correct, I should have a 19 tooth gear.  Based on this information, what gear (tooth count or color) should I have to get a correct reading?  transmission is a C6 with a 7 tooth drive gear in the transmission.
The 15 inch dia tires may be part of your situation.
1. Put the formula up that you are using.
2. Measure the diameter of the tire across (horizontally).
Jim
I promise to be politically correct in all my posts to keep the BBBB from vociferating.

Offline DougDyar

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Re: correct speedometer gear to get
« Reply #4 on: April 21, 2021, 09:28:31 PM »
Forget the formula. You have all the info you need to calculate the correct gear: Indicated speed, actual speed, and driven gear tooth count.

You're reading 60 when it's really 52. So you need to decrease the tooth count on the driven gear.

60-52=8
8/60=13% too fast

So you need to decrease the tooth count on the driven gear by 13%

How many teeth are on the driven gear now? If 19, then try a 17-tooth gear.
19x0.13=2.47 (call it 2)
19-2=17
« Last Edit: April 21, 2021, 09:46:21 PM by DougDyar »
0F02G203277
June 22, 1970 Dearborn BOSS 302

Offline RoyceP

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Re: correct speedometer gear to get
« Reply #5 on: April 21, 2021, 09:52:41 PM »

Well done!

Forget the formula. You have all the info you need to calculate the correct gear: Indicated speed, actual speed, and driven gear tooth count.

You're reading 60 when it's really 52. So you need to decrease the tooth count on the driven gear.

60-52=8
8/60=13% too fast

So you need to decrease the tooth count on the driven gear by 13%

How many teeth are on the driven gear now? If 19, then try a 17-tooth gear.
19x0.13=2.47 (call it 2)
19-2=17
1968 W code 427 Cougar XR-7 GTE Feb 23 Dearborn C6 / 3.50 open
1968 R code 428CJ Cougar XR-7 May 13 Dearborn C6 / 3.91 T - Lock

Offline sflaten

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Re: correct speedometer gear to get
« Reply #6 on: April 21, 2021, 10:10:12 PM »
Here is a place to calculate. Might need to check Tire Rack on actual run out of your tires
https://www.tciauto.com/speedometer-gear-calculator
Steve
1969 Mach 1, Champagne Gold
Dearborn built 3/4/69
351W 4V, PS, Power disc brakes
past ownership; 69 Mach 1(red), 79 Pace car, 69 Mach 1 (gold), 89 GT, 2011 GT500 (still own)