Speedo help please - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ratfreak8871 · 07-29-2018, 12:16 PM

79 c10 th400. have green 20tooth drive gear and white 36 tooth driven gear. 27in tire 3.73 rear. calculator says I need 54 tooth driven. every that sells them only go to 45 tooth. how hard is it to change the drive gear?

geezer#99 · 07-29-2018, 02:13 PM

Changing the drive gear can be a lot of fun.
You gotta drop the driveshaft, tranny crossmember and remove the tailhousing of the tranny.
Then pull the gear, swap it out and put it all back togethor.
I’ve always cheated.
Bypassed all that work completely.
I use a ratio adaptor that fits onto your speedo cable.
Like here. They have calculators too.

https://transmissioncenter.net/?swoo...-gears&paged=1

**hatzie** · 07-29-2018, 05:26 PM

Ratio adapters wear out faster than plastic gears.
I dont trust the calculators either.
The GM tables are in the parts books. You need your tire size, axle ratio, and transmission gives you the correct gears.

WilliamFd55 · 07-30-2018, 12:48 PM

Not Sure if you are looking to stay 100% stock? I went with a GPS Speedo when I did an LS swap. Very easy, plug and play. Eliminates speedo cable and you wouldn't have to worry about tire size or gears. Just have to modify gauge cluster.

Jake Wade · 07-30-2018, 07:05 PM

Now that is cool.

07-29-2018, 12:16 PM	#1
ratfreak8871 Registered User Join Date: May 2007 Location: ALTON, IL. Posts: 98	Speedo help please 79 c10 th400. have green 20tooth drive gear and white 36 tooth driven gear. 27in tire 3.73 rear. calculator says I need 54 tooth driven. every that sells them only go to 45 tooth. how hard is it to change the drive gear?

07-29-2018, 02:13 PM	#2
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,750	Re: Speedo help please Changing the drive gear can be a lot of fun. You gotta drop the driveshaft, tranny crossmember and remove the tailhousing of the tranny. Then pull the gear, swap it out and put it all back togethor. I’ve always cheated. Bypassed all that work completely. I use a ratio adaptor that fits onto your speedo cable. Like here. They have calculators too. https://transmissioncenter.net/?swoo...-gears&paged=1

07-29-2018, 05:26 PM	#3
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 5,014	Re: Speedo help please Ratio adapters wear out faster than plastic gears. I dont trust the calculators either. The GM tables are in the parts books. You need your tire size, axle ratio, and transmission gives you the correct gears. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.

07-30-2018, 12:48 PM	#4
WilliamFd55 Registered User Join Date: Dec 2016 Location: Glen Rock, NJ Posts: 258	Re: Speedo help please Not Sure if you are looking to stay 100% stock? I went with a GPS Speedo when I did an LS swap. Very easy, plug and play. Eliminates speedo cable and you wouldn't have to worry about tire size or gears. Just have to modify gauge cluster. Attached Images

07-30-2018, 07:05 PM	#5
Jake Wade Registered User Join Date: Aug 2005 Location: Louisiana Posts: 711	Re: Speedo help please Now that is cool.

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