Calculating Transmission plastic Gear - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Astro-Balls · 08-05-2020, 12:37 PM

How to determine proper speedo gear?
Need to change out the transmission speedo cable drive gear. NP435
Just replaced the rear end to a 3:54 Eaton TruTrak and now speedo is off.
Tire size is P255/75 R16

**MJN** · 08-05-2020, 12:52 PM

Check with the guys at transmissioncenter.net , they'll either have the correct gear or you'll need the ration adapter gearbox to correct your speedometer. They custom built one for me and its dead on.
https://transmissioncenter.net/?prod...edometer-gears

Astro-Balls · 08-05-2020, 01:01 PM

Thanks very much, Will Do.

**Boog** · 08-05-2020, 01:14 PM

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

notsolo · 08-05-2020, 01:25 PM

Also need Color, or number of teeth on drive gear.

my67c20 · 08-05-2020, 06:03 PM

By a whole set and mix and match until you get close enough to a GPS. I tried the calculators and asking others and in the end it was trial and error and close enough. Of course my speedo cable snapped a few weeks ago, so not its anyones guess how fast i am going

**kwmech** · 08-05-2020, 06:33 PM

Quote:

Originally Posted by Astro-Balls

How to determine proper speedo gear?
Need to change out the transmission speedo cable drive gear. NP435
Just replaced the rear end to a 3:54 Eaton TruTrak and now speedo is off.
Tire size is P255/75 R16

What ratio did you swap from?

OregonNed · 08-05-2020, 07:06 PM

Quote:

Originally Posted by my67c20

By a whole set and mix and match until you get close enough to a GPS. I tried the calculators and asking others and in the end it was trial and error and close enough. Of course my speedo cable snapped a few weeks ago, so not its anyones guess how fast i am going

I did this and got it right on the second speedo gear I tried.

Astro-Balls · 08-05-2020, 08:50 PM

Quote:

Originally Posted by kwmech

What ratio did you swap from?

Previously had 4.56

**Boog** · 08-05-2020, 11:33 PM

ten
Mitch someone posted this a while back.

**Bob B.** · 08-07-2020, 02:42 AM

Wait a minute, if you have a NP435, they DO NOT use the colored plastic gears that a Hydra-Matic and most GM manual transmissions use. This is not a GM manufactured transmission. NP435's have a drive (transmission output shaft) worm gear and a corresponding metal driven pinion gear/cable adapter assembly that must be changed in pairs together to get the proper ratio. Don't bother looking, they are all obsolete and there were DOZENS of combinations. Best way to correct for an axle ratio change is to use a mechanical ratio adapter gearbox as MJN pointed out and leave the existing drive and driven gears alone. They are available from several vendors, you provide them with information like how fast does the speedometer say you are going when you are actually going 60 (easy to verify with GPS). That way you will even correct for whatever tire size you are using.

Astro-Balls · 08-07-2020, 10:23 PM

Thanks very much, I have contacted https://transmissioncenter.net/?prod...edometer-gears
awaiting response.

Astro-Balls · 09-25-2020, 11:47 AM

Quote:

Originally Posted by Bob B.

Wait a minute, if you have a NP435, they DO NOT use the colored plastic gears that a Hydra-Matic and most GM manual transmissions use. This is not a GM manufactured transmission. NP435's have a drive (transmission output shaft) worm gear and a corresponding metal driven pinion gear/cable adapter assembly that must be changed in pairs together to get the proper ratio. Don't bother looking, they are all obsolete and there were DOZENS of combinations. Best way to correct for an axle ratio change is to use a mechanical ratio adapter gearbox as MJN pointed out and leave the existing drive and driven gears alone. They are available from several vendors, you provide them with information like how fast does the speedometer say you are going when you are actually going 60 (easy to verify with GPS). That way you will even correct for whatever tire size you are using.

Thanks, good info,
Do you know what vendors would carry a mechanical ratio adapter?

geezer#99 · 09-25-2020, 12:04 PM

It’s right there in post #2.

https://transmissioncenter.net/home/

Astro-Balls · 09-25-2020, 07:04 PM

Contacted them, No help.

geezer#99 · 09-25-2020, 07:13 PM

One like this.

https://transmissioncenter.net/shop/...ratio-adapter/

Or they don’t have one for your transmission.

08-05-2020, 12:37 PM	#1
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	Calculating Transmission plastic Gear How to determine proper speedo gear? Need to change out the transmission speedo cable drive gear. NP435 Just replaced the rear end to a 3:54 Eaton TruTrak and now speedo is off. Tire size is P255/75 R16 __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

08-05-2020, 01:01 PM	#3
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	Re: Calculating Transmission plastic Gear Thanks very much, Will Do. __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

08-05-2020, 01:14 PM	#4
Boog laying low Join Date: Mar 2001 Location: Searcy, Ark. USA Posts: 12,957	Re: Calculating Transmission plastic Gear https://www.tciauto.com/speedometer-gear-calculator __________________ Boog 69 Chevy stepside, 358/T350, 4.11 posi, 4.5/4 drop, rallys, poboy driver primer is finer 91 Chevy sportside, Tahoe, Yukon & GMC Crewcab All GM..'nuff said. I stand for the flag and kneel at the cross

08-05-2020, 01:25 PM	#5
notsolo Registered User Join Date: Apr 2008 Location: Md Posts: 2,451	Re: Calculating Transmission plastic Gear Also need Color, or number of teeth on drive gear. __________________ http://67-72chevytrucks.com/vboard/s...d.php?t=635605

08-05-2020, 06:03 PM	#6
my67c20 Registered User Join Date: Jun 2002 Location: Tucson AZ by way of WI & CA Posts: 507	Re: Calculating Transmission plastic Gear By a whole set and mix and match until you get close enough to a GPS. I tried the calculators and asking others and in the end it was trial and error and close enough. Of course my speedo cable snapped a few weeks ago, so not its anyones guess how fast i am going

08-05-2020, 12:52 PM	#2
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 2,963	Re: Calculating Transmission plastic Gear Check with the guys at transmissioncenter.net , they'll either have the correct gear or you'll need the ration adapter gearbox to correct your speedometer. They custom built one for me and its dead on. https://transmissioncenter.net/?prod...edometer-gears __________________ '51 South Bend Model "9A" '56 Chevy 3600 NAPCO '74 Chevy Cheyenne Super K20-restored on 37's '72 GMC Sierra Custom K2500- in progress '08 GMC Envoy Denali '12 GMC Sierra 2500HD Denali '17 GMC Yukon XL VCCA member #58596 http://www.nohrco.com

08-05-2020, 11:33 PM	#10
Boog laying low Join Date: Mar 2001 Location: Searcy, Ark. USA Posts: 12,957	Re: Calculating Transmission plastic Gear ten Mitch someone posted this a while back. Attached Images __________________ Boog 69 Chevy stepside, 358/T350, 4.11 posi, 4.5/4 drop, rallys, poboy driver primer is finer 91 Chevy sportside, Tahoe, Yukon & GMC Crewcab All GM..'nuff said. I stand for the flag and kneel at the cross

08-07-2020, 02:42 AM	#11
Bob B. Senior Member Join Date: Aug 2005 Location: Los Angeles Posts: 1,308	Re: Calculating Transmission plastic Gear Wait a minute, if you have a NP435, they DO NOT use the colored plastic gears that a Hydra-Matic and most GM manual transmissions use. This is not a GM manufactured transmission. NP435's have a drive (transmission output shaft) worm gear and a corresponding metal driven pinion gear/cable adapter assembly that must be changed in pairs together to get the proper ratio. Don't bother looking, they are all obsolete and there were DOZENS of combinations. Best way to correct for an axle ratio change is to use a mechanical ratio adapter gearbox as MJN pointed out and leave the existing drive and driven gears alone. They are available from several vendors, you provide them with information like how fast does the speedometer say you are going when you are actually going 60 (easy to verify with GPS). That way you will even correct for whatever tire size you are using. __________________ 1967 GMC CM-2500 Camper Cruiser, 351E V-6, NP 435 4 speed, Dana 60, and factory A/C. 2012 GMC K-3500 WT regular cab, 6.0L Vortec, 6L90.

08-07-2020, 10:23 PM	#12
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	Re: Calculating Transmission plastic Gear Thanks very much, I have contacted https://transmissioncenter.net/?prod...edometer-gears awaiting response. __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

09-25-2020, 12:04 PM	#14
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,512	Re: Calculating Transmission plastic Gear It’s right there in post #2. https://transmissioncenter.net/home/

09-25-2020, 07:04 PM	#15
Astro-Balls Registered User Join Date: Feb 2019 Location: Nor Cal Posts: 822	Re: Calculating Transmission plastic Gear Contacted them, No help. __________________ 1968 C20 Fleetside 396 4sp PS PB AC CST . https://www.youtube.com/watch?v=RWK8GgWD4uA

09-25-2020, 07:13 PM	#16
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,512	Re: Calculating Transmission plastic Gear One like this. https://transmissioncenter.net/shop/...ratio-adapter/ Or they don’t have one for your transmission.



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