checking gear ratio - The 1947 - Present Chevrolet & GMC Truck Message Board Network

70c10 · 08-22-2002, 08:05 AM

How do you check the gear ratio by spinning the tire and counting revolutions on driveshaft?. I tried it on my project truck and got 1.5 revs on the driveshaft. I did it 3 times. Am I supposed to multiplt that number by something?. Thanks

**Yukon Jack** · 08-22-2002, 09:24 AM

I think on an open diff it must not work cause I tried that two nights ago to try to figure out the ratio of the rearend in the 1 ton crew parts truck I recently bought. Only one wheel was spinning and it was 2 driveshaft revolutions for 1 wheel revolution. Guess I will be pulling the cover one of these evenings to find out.

Canada20 · 08-22-2002, 10:07 AM

I think I heard that on an open diff you need to immobilize the wheel you're not spinning for that method to work. Kind of makes sense to me but obviously I've never tried it

Greg

71GMC_3/4T · 08-22-2002, 10:10 AM

If you rotated the one wheel one turn, the driveshaft would travel half the normal gear ratio.
Use a piece of chalk to mark the wheel (unless the neighborhood dogs have done it for you already). Turn the wheel exactly *two* turns. Count the number of driveshaft revs and that will be your gear ratio. You may have to guess approximately how much past a whole number it goes to get the number past the decimal point, but you should be able to wing it knowing the common ratios. Also, be sure to take up any backlash by rotating the wheel slowly and at least 1/2 turn before starting to count.

70c10 · 08-22-2002, 10:50 AM

71GMC,
You are correct. It was actually a hair past 1.5. I know it has a 3:07 rear so that makes sense. Thanks!

Blazer1970 · 08-22-2002, 11:22 AM

Yup, and I would be willing to bet that Yukon Jack has 4.10s.

08-22-2002, 08:05 AM	#1
70c10 67-72 Addict! Join Date: Feb 2002 Location: The TAX State! Posts: 7,857	checking gear ratio How do you check the gear ratio by spinning the tire and counting revolutions on driveshaft?. I tried it on my project truck and got 1.5 revs on the driveshaft. I did it 3 times. Am I supposed to multiplt that number by something?. Thanks __________________ Jim 1970 C/10 Fleetside w/Ghost Flames Lowered 4.5" front and 4" rear (Raked) 355/350 Turbo w/shift kit 10" Redneck Performance Verter w/2500 stall Hooker Super Comps part#2808-1 Performer RPM Air-Gap 12 Bolt w/3:73 gears- Eaton Posi Comp Cams XE262 with 1.6 Crane Energizers, Road Demon 625 and Brodix IK 180 heads

08-22-2002, 09:24 AM	#2
Yukon Jack Post Whore Join Date: Feb 2000 Location: Rose Hill, KS, USA Posts: 12,683	I think on an open diff it must not work cause I tried that two nights ago to try to figure out the ratio of the rearend in the 1 ton crew parts truck I recently bought. Only one wheel was spinning and it was 2 driveshaft revolutions for 1 wheel revolution. Guess I will be pulling the cover one of these evenings to find out. __________________ 1970 Blazer with a 400 sbc and 4" lift 1980 Pontiac Trans Am, 455 Oldsmobile 2012 Kawasaki Concours 14

08-22-2002, 10:10 AM	#4
71GMC_3/4T Weapons Of Construction Join Date: Apr 2000 Location: Colorado, USA Posts: 1,095	If you rotated the one wheel one turn, the driveshaft would travel half the normal gear ratio. Use a piece of chalk to mark the wheel (unless the neighborhood dogs have done it for you already). Turn the wheel exactly two turns. Count the number of driveshaft revs and that will be your gear ratio. You may have to guess approximately how much past a whole number it goes to get the number past the decimal point, but you should be able to wing it knowing the common ratios. Also, be sure to take up any backlash by rotating the wheel slowly and at least 1/2 turn before starting to count. __________________ 1971 GMC 2500, 402/TH400 4.10 Daily Driver Lafayette, CO

08-22-2002, 10:50 AM	#5
70c10 67-72 Addict! Join Date: Feb 2002 Location: The TAX State! Posts: 7,857	71GMC, You are correct. It was actually a hair past 1.5. I know it has a 3:07 rear so that makes sense. Thanks! __________________ Jim 1970 C/10 Fleetside w/Ghost Flames Lowered 4.5" front and 4" rear (Raked) 355/350 Turbo w/shift kit 10" Redneck Performance Verter w/2500 stall Hooker Super Comps part#2808-1 Performer RPM Air-Gap 12 Bolt w/3:73 gears- Eaton Posi Comp Cams XE262 with 1.6 Crane Energizers, Road Demon 625 and Brodix IK 180 heads

08-22-2002, 11:22 AM	#6
Blazer1970 Registered User Join Date: Dec 1999 Location: Old Mission, MI, USA Posts: 2,510	Yup, and I would be willing to bet that Yukon Jack has 4.10s. __________________ Tim

08-22-2002, 10:07 AM	#3
Canada20 Registered User Join Date: Jun 2001 Location: Calgary, Alberta, Canada Posts: 802	I think I heard that on an open diff you need to immobilize the wheel you're not spinning for that method to work. Kind of makes sense to me but obviously I've never tried it Greg



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