decoding the 64-66 1/2 ton Rear end gear ratio - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**GMCNUT** · 08-16-2014, 05:52 PM

Hey guys - I have a "known to be good" spare 1/2 ton rear end I saved for a rainy day and I am building a 66 short fleet, so I decided to pull the inspection plate and see if I could count the teeth or something to determine the ratio....I could not tell if I was counting the teeth right, so I snapped a picture of the stampings on the ring gear and decided to sling this info at you guys and see what you can tell me? not even sure these markings tell us anything, but willing to try! See pictures below.

Hopefully you can see these markings clearly, but in case you cannot, starting from left to right, is the GM pt # 3817778 followed by an 11 then a 41GM then a 3 then what looks like a date of 64 1 27. on the opposite side of the ring gear it says 2-18.

If this info does not tell us the gear ratio, please feel free to suggest something. My next idea was to wipe a little white paint on one of the teeth and turn the ring gear and try to get an accurate count that way. As for counting the pinion gear teeth, I have no idea how to do that since it sits deep behind the carrier and everything - may have to remove it to tell. Thoughts? Thanks, Steve

66Submarine · 08-16-2014, 06:33 PM

11 - 41 = 41/11=3.73

**jayoldschool** · 08-16-2014, 09:26 PM

Yes, factory gears are easy to do the math on.

**GMCNUT** · 08-17-2014, 09:02 AM

Quote:

Originally Posted by 66Submarine

11 - 41 = 41/11=3.73

Awesome - thank you. Never had a reason to know that was how you did it....thanks for the info!

08-16-2014, 05:52 PM	#1
GMCNUT Senior Member Join Date: Jun 2007 Location: Arab Alabama Posts: 1,809	decoding the 64-66 1/2 ton Rear end gear ratio Hey guys - I have a "known to be good" spare 1/2 ton rear end I saved for a rainy day and I am building a 66 short fleet, so I decided to pull the inspection plate and see if I could count the teeth or something to determine the ratio....I could not tell if I was counting the teeth right, so I snapped a picture of the stampings on the ring gear and decided to sling this info at you guys and see what you can tell me? not even sure these markings tell us anything, but willing to try! See pictures below. Hopefully you can see these markings clearly, but in case you cannot, starting from left to right, is the GM pt # 3817778 followed by an 11 then a 41GM then a 3 then what looks like a date of 64 1 27. on the opposite side of the ring gear it says 2-18. If this info does not tell us the gear ratio, please feel free to suggest something. My next idea was to wipe a little white paint on one of the teeth and turn the ring gear and try to get an accurate count that way. As for counting the pinion gear teeth, I have no idea how to do that since it sits deep behind the carrier and everything - may have to remove it to tell. Thoughts? Thanks, Steve Attached Images __________________ My Projects and Toys - All 100% Stock 1972 CST "Highlander" Blazer - 350 auto, PS, PB, A/C, Tilt, Tach, AM/FM & Positraction 1966 Chevy SWB Fleetside BBW Custom Cab, #'s matching 327 V8 & Powerglide, factory deluxe heat, PS, PB, Tinted Glass, sway bar 1961 Chevy SWB Fleetside BBW Custom Cab #'s matching 283 V8 & Powerglide w/ uber-rare Cool-Pack A/C, factory PB, deluxe heat, Clock 1961 GMC K1000 4x4 SWB w/ BBW, Custom Cab and numbers matching 305D V6 engine & 4 spd 1960 GMC SWB Fleetside BBW Custom Cab #'s matching 305A V6 & Hydramatic, factory PB, Clock, Tach, Compass, Hood Jet 1959 Chevy SWB Fleetside BBW Custom Cab w/ rare stainless bed trim, #'s matching 283 V8 & Hydramatic, Deluxe Heat, radio, spotlight 1958 Chevy Cameo #'s matching 283 V8 & Hydramatic, radio, window washers, Factory PB 1958 GMC BBW SWB Fleetside Custom Cab, #'s matching 336 V8 Hydramatic, Factory PS, PB, Clock, RSD, Deluxe Heat & Hood Jet 1957 GMC Suburban Pickup (GMC's Cameo) #'s matching 347 V8 Hydramatic, uber-rare vacuum ashtray, Factory PS, PB, Clock, RSD, Hood Jet 1956 Chevy 3800 1 ton w/ rare 9 ft stepside bed, factory 265 V8 w/ Hydramatic, factory PB, Custom Cab, deluxe heat, spotlight and hood jet.

08-16-2014, 06:33 PM	#2
66Submarine Registered User Join Date: Sep 2012 Location: Dallas, GA Posts: 1,497	Re: decoding the 64-66 1/2 ton Rear end gear ratio 11 - 41 = 41/11=3.73 __________________ 1965 C30 pickup 350/SM420/4.10's (daily driver) thread 1968 Impala 4 door sedan (future driver project) thread

08-16-2014, 09:26 PM	#3
jayoldschool Moderator Join Date: Dec 2012 Location: ON, Canada Posts: 2,176	Re: decoding the 64-66 1/2 ton Rear end gear ratio Yes, factory gears are easy to do the math on.



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