How do you calculate your gear ratio? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

69GMC910 · 08-23-2010, 08:56 PM

My old mans truck revs at 3200 or something @ 65mph with a 28" tire... When he jacks the back end up and gives the wheel a turn, the driveshaft only turns about 1.8 times... I ran his numbers through a calculator and it said 4.10, so why does the driveshaft only turn 1.8ish times?

71meangreenc10 · 08-23-2010, 09:05 PM

Nevermind...... Cant remember, I just pop the cover and count the gears....

Smitty

c10seconds · 08-23-2010, 09:38 PM

Gotta take the diff cover off. There are two numbers stamped side by side on the ring gear. Divide the big one by the small one and that should be you ratio.

raycow · 08-24-2010, 03:52 PM

You don't have to take off the cover unless you really enjoy draining the oil, getting yourself greasy, scraping off the old gasket, etc.

Put matching marks on the pinion yoke and differential housing. Jack ONE rear wheel off the ground, put the transmission in neutral, and the parking brake off. Turn the wheel exactly TWO revolutions and count how many times the driveshaft turns, estimating any fractional turn as closely as possible. That number is your axle ratio.

If the wheel is difficult or impossible to turn, you may have posi. In that case, jack both rear wheels off the ground and turn either of them only ONE revolution. Count the driveshaft turns the same way as for an open rear.

Ray

cdowns · 08-24-2010, 04:13 PM

draining 40year old rearend grease cleaning and replacing with new grease is a worthwhile saturday project

08-23-2010, 08:56 PM	#1
69GMC910 MOTORHEAD! Join Date: May 2000 Location: Devon AB, Canada Posts: 849	How do you calculate your gear ratio? My old mans truck revs at 3200 or something @ 65mph with a 28" tire... When he jacks the back end up and gives the wheel a turn, the driveshaft only turns about 1.8 times... I ran his numbers through a calculator and it said 4.10, so why does the driveshaft only turn 1.8ish times? __________________ 82 GMC High Sierra VortecPro 496 12 Bolt, 3.42's AFR 265 Holley 850 Performer RPM Air Gap Hooker 2455's TCI Turbo 400 Super Street Fighter TRRC 12 Bolt Eaton Posi Mark Williams Axles Cal-Tracs Hoosier 30x10.5 Radial Slick Full weight long box pickup Current best- 11.33@117mph

08-23-2010, 09:05 PM	#2
71meangreenc10 Its a Truck Thing...... Join Date: Dec 2004 Location: Sierra Vista, Arizona Posts: 3,159	Re: How do you calculate your gear ratio? Nevermind...... Cant remember, I just pop the cover and count the gears.... Smitty __________________ 71 C10 283/3SPD Full Resto 71 GMC 1500 Sierra Grande http://67-72chevytrucks.com/vboard/s...d.php?t=518599 70 C10 Suburban Former military GSA truck. 72 Chevy Blazer 4X4, Sloppy Jo, Mountain Climber. Wife says no more trucks. http://67-72chevytrucks.com/vboard/s...d.php?t=707378 72 GMC 1 Ton Motor Home, wife said no more trucks until she saw this one. Gen 3 6.0/4L80E 4.10 http://67-72chevytrucks.com/vboard/s...d.php?t=761110 68 GMC Suburban was 3/4 ton, now 1/2.Wife shook her head 71 C30 Wrecker 71 C20 Scott-Bilt As weird as it gets..BB Cheyenne AC Truck 68 GMC Long Stepside. They keep following me home 71 C20 Longhorn/Ole Yellor 69 C30 Former Motor Home, Flat Bed time Last edited by 71meangreenc10; 08-23-2010 at 09:16 PM.

08-23-2010, 09:38 PM	#3
c10seconds Registered User Join Date: Oct 2009 Location: gilbert,AZ Posts: 947	Re: How do you calculate your gear ratio? Gotta take the diff cover off. There are two numbers stamped side by side on the ring gear. Divide the big one by the small one and that should be you ratio.

08-24-2010, 03:52 PM	#4
raycow Registered User Join Date: Mar 2003 Location: San Francisco, CA USA Posts: 2,454	Re: How do you calculate your gear ratio? You don't have to take off the cover unless you really enjoy draining the oil, getting yourself greasy, scraping off the old gasket, etc. Put matching marks on the pinion yoke and differential housing. Jack ONE rear wheel off the ground, put the transmission in neutral, and the parking brake off. Turn the wheel exactly TWO revolutions and count how many times the driveshaft turns, estimating any fractional turn as closely as possible. That number is your axle ratio. If the wheel is difficult or impossible to turn, you may have posi. In that case, jack both rear wheels off the ground and turn either of them only ONE revolution. Count the driveshaft turns the same way as for an open rear. Ray Last edited by raycow; 08-24-2010 at 03:54 PM.

08-24-2010, 04:13 PM	#5
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: How do you calculate your gear ratio? draining 40year old rearend grease cleaning and replacing with new grease is a worthwhile saturday project __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY



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