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Restoring - General discussions that span across many different groups of years and models => Drivetrain => Topic started by: truetriplex on April 21, 2021, 10:41:04 AM

Title: correct speedometer gear to get
Post by: truetriplex 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.

Title: Re: correct speedometer gear to get
Post by: RoyceP 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.

Title: Re: correct speedometer gear to get
Post by: 67gta289 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

Title: Re: correct speedometer gear to get
Post by: jwc66k on April 21, 2021, 05:44:55 PM

Quote from: truetriplex 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.

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

Title: Re: correct speedometer gear to get
Post by: DougDyar 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

Title: Re: correct speedometer gear to get
Post by: RoyceP on April 21, 2021, 09:52:41 PM

Well done!

Quote from: DougDyar 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

Title: Re: correct speedometer gear to get
Post by: sflaten 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