Speedometer correction for tires - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Eagle Kammback · 09-17-2023, 08:12 AM

I know this has been covered a lot, but so many links are just dead that could help me.
I have a 2wd 72 C25, SM465, 4 10 Dana 60
I am tired of the extremely wonky speedometer needle, so I wanna replace the cable (at least im told that's from the cable)
Firstly, do you know what length cable I need?
Secondly, I'm running 285/75R16s in the back, what gear should I have?
She originally had 9.5R16.5s when I got her.
Thanks!

**Ironangel** · 09-17-2023, 09:45 AM

Cant help with the gears, but if you take your existing speedo cable out and clean it, you'll most likely solve the wavey or twitchy speedo needle. You can pull the inner cable out of the outer sheath and soak them in a parts cleaner. Or a pan and some gasoline or a couple cans of break clean. Once clean, squirt some white lithium grease into the sheath and reassemble the inner into the outer.

**Getter-Done** · 09-17-2023, 09:57 AM

GMCPauls has one that is 70"

Scroll down about 5th from the bottom.

Link: https://gmcpauls.com/instrument_panel.htm

**bowtie72** · 09-17-2023, 10:30 AM

Like said before start with the cable. You would be better off cleaning the old one and lubing instead of buying a Chinese cable. Speedo is corrected by the cable drive gears in the trans or a gear reducer on the trans. You should be able to find info how to calculate and what color gear you need

**pjmoreland** · 09-17-2023, 11:24 AM

Here's a Speedometer gear calculator you can use. It says your tire diameter is 32.83. Plug that in, along with 4.10 for your rear end. You will have to determine the number of teeth on your currently installed drive gear so you can enter that, too. Then it will calculate the number of teeth you need to get for the driven gear.

https://www.tciauto.com/speedometer-gear-calculator

Eagle Kammback · 09-17-2023, 05:06 PM

Awesome! Thanks!
The drive teeth would be the one on the transmission side?

**pjmoreland** · 09-17-2023, 05:17 PM

Yes. The drive gear is mounted inside the transmission on the main output shaft going to your driveshaft.

Orange Crate · 09-19-2023, 03:20 PM

https://transmissioncenter.net/produ...dometer-gears/

Eagle Kammback · 09-19-2023, 05:53 PM

Quote:

Originally Posted by bowtie72

Like said before start with the cable. You would be better off cleaning the old one and lubing instead of buying a Chinese cable. Speedo is corrected by the cable drive gears in the trans or a gear reducer on the trans. You should be able to find info how to calculate and what color gear you need

Pulled the cable, from the speedometer side as you're supposed to, and the cable tip broke off, dang it.
Can you buy just that piece and crimp it on?

09-17-2023, 08:12 AM	#1
Eagle Kammback Registered User Join Date: Jan 2017 Location: Indianapolis Posts: 25	Speedometer correction for tires I know this has been covered a lot, but so many links are just dead that could help me. I have a 2wd 72 C25, SM465, 4 10 Dana 60 I am tired of the extremely wonky speedometer needle, so I wanna replace the cable (at least im told that's from the cable) Firstly, do you know what length cable I need? Secondly, I'm running 285/75R16s in the back, what gear should I have? She originally had 9.5R16.5s when I got her. Thanks! __________________ 1972 GMC C25 350 Muncie 465 - Daily Driver 1979 AMC AMX 304 4 Speed -Mechanic Special 1983 AMC Eagle - The Eternal Project

09-17-2023, 09:45 AM	#2
Ironangel Senior Member Join Date: Feb 2010 Location: Falls City, Nebraska "100 Miles From Nowhere" Posts: 2,219	Re: Speedometer correction for tires Cant help with the gears, but if you take your existing speedo cable out and clean it, you'll most likely solve the wavey or twitchy speedo needle. You can pull the inner cable out of the outer sheath and soak them in a parts cleaner. Or a pan and some gasoline or a couple cans of break clean. Once clean, squirt some white lithium grease into the sheath and reassemble the inner into the outer. __________________ Michael of the clan Hill, "Two Seventy Two's" 71 1-ton Dually 350 4-Speed 71 C/50 Grain Truck, 350 Split-Axle 4-Speed 02 3/4 ton Express 14 Indian Chief Vintage 1952 Ford 8N, "Only Ford Allowed On The Property" "Be American, Buy American"

09-17-2023, 09:57 AM	#3
Getter-Done Senior Member Join Date: Dec 2009 Location: TN. Posts: 8,491	Re: Speedometer correction for tires GMCPauls has one that is 70" Scroll down about 5th from the bottom. Link: https://gmcpauls.com/instrument_panel.htm __________________ ________________________________________________________________________________________ ____________________________________________ 84 Chevy K-20 63 Impala (my high school car) http://67-72chevytrucks.com/vboard/s...Crew Cab Build

09-17-2023, 10:30 AM	#4
bowtie72 Registered User Join Date: Mar 2007 Location: Colorado Posts: 363	Re: Speedometer correction for tires Like said before start with the cable. You would be better off cleaning the old one and lubing instead of buying a Chinese cable. Speedo is corrected by the cable drive gears in the trans or a gear reducer on the trans. You should be able to find info how to calculate and what color gear you need __________________ 1968 SWB "Little White Truck" 1972 LWB LS 4l80 "Plain Jane!!"

09-17-2023, 11:24 AM	#5
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,066	Re: Speedometer correction for tires Here's a Speedometer gear calculator you can use. It says your tire diameter is 32.83. Plug that in, along with 4.10 for your rear end. You will have to determine the number of teeth on your currently installed drive gear so you can enter that, too. Then it will calculate the number of teeth you need to get for the driven gear. https://www.tciauto.com/speedometer-gear-calculator

09-17-2023, 05:06 PM	#6
Eagle Kammback Registered User Join Date: Jan 2017 Location: Indianapolis Posts: 25	Re: Speedometer correction for tires Awesome! Thanks! The drive teeth would be the one on the transmission side? __________________ 1972 GMC C25 350 Muncie 465 - Daily Driver 1979 AMC AMX 304 4 Speed -Mechanic Special 1983 AMC Eagle - The Eternal Project

09-17-2023, 05:17 PM	#7
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,066	Re: Speedometer correction for tires Yes. The drive gear is mounted inside the transmission on the main output shaft going to your driveshaft.

09-19-2023, 03:20 PM	#8
Orange Crate Registered User Join Date: Feb 2011 Location: Under the Truck Dreaming Posts: 765	Re: Speedometer correction for tires https://transmissioncenter.net/produ...dometer-gears/ __________________ 72 K20 Custom Camper, D60, 14BFF, full roller 406 TPI w custom built ECU and yes..it has tow hooks...and a 5 speed



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