Gear ratios - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mister.freeze · 10-11-2007, 01:10 PM

I was looking at an axle setup for sale somebody posted and they were 3.08 gears. Isn't that high? I'm guessing 3.73 is standard for a 4x4, and I did the math and 3.35 would gain me 9% gass mileage. I've got a stroker in the Fox, and figure I can go with higher gears, but how high is too high?

**1971K54X4** · 10-11-2007, 01:38 PM

Mine came with 3.07's

**Yukon Jack** · 10-11-2007, 02:12 PM

My GM restoration pack shows for the 2wd K5 that 3.73 was standard for the 250 six regardless of transmission and standard with the 307 V8 with a three speed.

3.07's were standard for the 307 V8 and 4 speed manual or auto, along with any 350 V8 combo.

4.11's were optional for all and if it had the 3.73's then 3.07 was an option and if it had 3.07's then 3.73's were also optional.

For the 4wd K5 3.73's were standard with all engines except for the 350 V8 which it shows only got 3.07's.

For whatever reason it doesn't show any optional gear ratios for the 4wd K5's.

**1970cstblazer** · 10-11-2007, 02:29 PM

My '70 has optional 3.73 gears.

earl87gta · 10-11-2007, 02:51 PM

my 71 blazer had 3:07 my 72 had 3:73 and my k20 burb had 4:57 rear 4:56 front.

bouncytruck · 10-11-2007, 04:41 PM

I've got the 3.07's in mine. My last K5 had 4.10's, but they were not stock. My first truck had 3.73's and I believe it was listed as an option on the glove box.

earl87gta · 10-11-2007, 04:45 PM

here is my favorate gear calculator http://www.grimmjeeper.com/gears.html

BLUE THUNDER · 10-11-2007, 07:38 PM

hey dude thanks for the parts, mario.

mister.freeze · 10-11-2007, 10:42 PM

I'd go from 9.16mpg w/3.73's to 10.74mpg w/3.09's.

You're welcome, Mario.

the_goat · 10-12-2007, 02:07 AM

so with 33's and 3.73 gears would I even use the 4th gear if I put in a 700r4? at 65 I would still be under 2000 rpms.

earl87gta · 10-12-2007, 07:43 AM

a stock 700r4 I think torqe converter locks up at like 1800 RPM so you would just be getting in to it. I know the truck and the fire bird and the vet all lock at diffrent RPMs the vet having the highest I think it is 2100 but its been a while since Ive read up on that so I may be off some.

Elwayno · 10-12-2007, 10:33 AM

You won't neccesarily get better gas mileage. Maybe on the highway with a stick but could be worse in town. Due to more slippage if you got an automatic it could be the same or really close. Just don't change it so much you have too much gear to pull the hills on the freeway. Mine is close now.

earl87gta · 10-12-2007, 04:59 PM

true but if he has a T350 now it would make a difrenc plus the 700r4 has a lower 1st gear so he could use a higher axle gear if he wanted to and wouldnt loose as much low end power as a t350 would.

Long Roof · 10-12-2007, 05:04 PM

My Jimmy was optioned with 3.73's

Elwayno · 10-12-2007, 05:18 PM

Earl if you change trannies and such things can be different. I was just pointing out that there is no free lunch so to speak. Not trying to be a naysayer. Gas mileage on the freeway is mostly a funtion of drag and driveline friction. In town is more to do with weight/inertia/gearing/lead foot. If going 60mph takes 50Hp with 3.73's it will take 50Hp with 3.07's. There may be very marginal gains because of the friction of the gears themselves but ignore that for a second. So if you change gears your engine slows down say 18%. Most likely your motor doesn't make as much power there so you have to push down on the gas harder. So you end up using the same gas. Now maybe you have a big block that makes a gob of torque everywhere, then maybe you don't have that problem, you lower rpm and get a gain in less loss from the part aoround the rings and piston that doesn't burn. Maybe you have a small block and you end up pushing the rpm down far enough you are off the torque peak completely and the engine is less efficient. Engines are most efficient at peak torque. As in the amount of gas used per Hp made goes down slightly so you don't want to get too far away from that. If it were jsut as ez as puttin higher gears I would get some 1.0:1's made and quadruple my gas mileage. We all know that won't work well.

earl87gta · 10-12-2007, 05:26 PM

I wasnt disagreeing with you. He didnt say what trans he had now but most guys that go to a 700r4 are swaping from a t350 wich is geared almost the same as a 700r4 so it wouldnt make that big of difrenc. Now if he had a sm465 or something I would think diffrent.

Jimmy72custom · 10-14-2007, 04:17 PM

I have a 72 Jimmy 350 / 465 / 307 / 33x12.50x15 / I run about 2200 rpm @ 65 MPH. I do notice the freeway hills. I have been thinking about a gear venders OD, to raise my 3rd gear to for the freeway hills. I have not checked my MPG, something I should do.

10-11-2007, 01:10 PM	#1
mister.freeze Registered User Join Date: Dec 2003 Location: Houston, TX Posts: 733	Gear ratios I was looking at an axle setup for sale somebody posted and they were 3.08 gears. Isn't that high? I'm guessing 3.73 is standard for a 4x4, and I did the math and 3.35 would gain me 9% gass mileage. I've got a stroker in the Fox, and figure I can go with higher gears, but how high is too high? __________________ '72 K5, known as 'the Fox.'

10-11-2007, 01:38 PM	#2
1971K54X4 Member Join Date: Jan 2007 Location: Manchester, Iowa Posts: 1,493	Re: Gear ratios Mine came with 3.07's __________________ Korey 1955 Bel Air 2 door hardtop 1971 Blazer 4x4 1973 Z-28 Prostreet

10-11-2007, 02:12 PM	#3
Yukon Jack Post Whore Join Date: Feb 2000 Location: Rose Hill, KS, USA Posts: 12,689	Re: Gear ratios My GM restoration pack shows for the 2wd K5 that 3.73 was standard for the 250 six regardless of transmission and standard with the 307 V8 with a three speed. 3.07's were standard for the 307 V8 and 4 speed manual or auto, along with any 350 V8 combo. 4.11's were optional for all and if it had the 3.73's then 3.07 was an option and if it had 3.07's then 3.73's were also optional. For the 4wd K5 3.73's were standard with all engines except for the 350 V8 which it shows only got 3.07's. For whatever reason it doesn't show any optional gear ratios for the 4wd K5's. __________________ 1970 Blazer with a 400 sbc and 4" lift 1980 Pontiac Trans Am, 455 Oldsmobile 2012 Kawasaki Concours 14

10-11-2007, 02:29 PM	#4
1970cstblazer All stock and staying that way Join Date: Jul 2003 Location: Elkland, PA Posts: 2,083	Re: Gear ratios My '70 has optional 3.73 gears. __________________ 1970 K/5 Blazer CST 4WD, TOTM winner December 2024! Medium Bronze poly, 93k ACT. miles, 350, 4 speed, rear positraction, 16.5" x 8.25" HD Kelsey Hayes wheel option, tilt, tach, vacuum, AM/FM, manual throttle...Dad ordered and purchased new 4/70. Currently frame off restoration finally getting close to completion.. 1970 K20, To be a clone of my Grandfather's K20, Dark Blue poly, 350, 4 speed, 16.5" X 8.25" Kelsey Hayes wheel option, tilt, tach, vacuum, AM radio. Latest project truck 1972 Cheyenne Super 20 2WD, Dark Blue/White, 90K ACT. miles, 402, TH400, 4.10 open, tilt, tach, vacuum, A/C, AM/FM, manual throttle.. A mostly original paint never rusted Texas survivor... 2017 Sierra 1500 SLT 4WD, Black, 46k miles, 5.3, 6L80E, 3.42 LS, 20" polished wheels, everything but moonroof and 6.2... 2019 Canyon SLT 4WD, White, 67k miles, 3.6, 8L45, 3.23 LS, 18" wheels

10-11-2007, 02:51 PM	#5
earl87gta Senior Member Join Date: Dec 2003 Location: Kingsport TN Posts: 4,641	Re: Gear ratios my 71 blazer had 3:07 my 72 had 3:73 and my k20 burb had 4:57 rear 4:56 front. __________________ Earl 68 2500 4x4 GMC Burb

10-11-2007, 04:41 PM	#6
bouncytruck SKINNY TIRES RULE! Join Date: Sep 2001 Location: Bothell, WA Posts: 11,038	Re: Gear ratios I've got the 3.07's in mine. My last K5 had 4.10's, but they were not stock. My first truck had 3.73's and I believe it was listed as an option on the glove box. __________________ bouncytruck 1972 K5 The Hauler 1994 K1500 Suburban The Daily2010 Kia Soul Wife's Ride2014 Fiat 500L

10-11-2007, 04:45 PM	#7
earl87gta Senior Member Join Date: Dec 2003 Location: Kingsport TN Posts: 4,641	Re: Gear ratios here is my favorate gear calculator http://www.grimmjeeper.com/gears.html __________________ Earl 68 2500 4x4 GMC Burb

10-11-2007, 07:38 PM	#8
BLUE THUNDER Registered User Join Date: May 2007 Location: San Antonio Posts: 993	Re: Gear ratios hey dude thanks for the parts, mario.

10-11-2007, 10:42 PM	#9
mister.freeze Registered User Join Date: Dec 2003 Location: Houston, TX Posts: 733	Re: Gear ratios I'd go from 9.16mpg w/3.73's to 10.74mpg w/3.09's. You're welcome, Mario. __________________ '72 K5, known as 'the Fox.'

10-12-2007, 02:07 AM	#10
the_goat Registered User Join Date: Mar 2007 Location: Carlsbad, Ca Posts: 385	Re: Gear ratios so with 33's and 3.73 gears would I even use the 4th gear if I put in a 700r4? at 65 I would still be under 2000 rpms.

10-12-2007, 07:43 AM	#11
earl87gta Senior Member Join Date: Dec 2003 Location: Kingsport TN Posts: 4,641	Re: Gear ratios a stock 700r4 I think torqe converter locks up at like 1800 RPM so you would just be getting in to it. I know the truck and the fire bird and the vet all lock at diffrent RPMs the vet having the highest I think it is 2100 but its been a while since Ive read up on that so I may be off some. __________________ Earl 68 2500 4x4 GMC Burb

10-12-2007, 10:33 AM	#12
Elwayno Registered User Join Date: Mar 2007 Location: Livermore, CA Posts: 34	Re: Gear ratios You won't neccesarily get better gas mileage. Maybe on the highway with a stick but could be worse in town. Due to more slippage if you got an automatic it could be the same or really close. Just don't change it so much you have too much gear to pull the hills on the freeway. Mine is close now.

10-12-2007, 04:59 PM	#13
earl87gta Senior Member Join Date: Dec 2003 Location: Kingsport TN Posts: 4,641	Re: Gear ratios true but if he has a T350 now it would make a difrenc plus the 700r4 has a lower 1st gear so he could use a higher axle gear if he wanted to and wouldnt loose as much low end power as a t350 would. __________________ Earl 68 2500 4x4 GMC Burb

10-12-2007, 05:04 PM	#14
Long Roof Old School, New Style Join Date: Jun 2005 Location: Monroeville, Ohio Posts: 790	Re: Gear ratios My Jimmy was optioned with 3.73's __________________ The Good: - 72 GMC Jimmy 4x4 - 307 cid / 350 th - 70 Chevy Suburban 1/2T 2wd - 454 cid /400 th - 96 Chevy Impala SS - mainly stock - 98 Chevy Tahoe LT 4x4 - all stock The Bad: - 86 Chevy Suburban 1/2T 4x4 - winter beater The Ugly: - 72 Chevy Suburban 1/2T 2wd - parts truck

10-12-2007, 05:18 PM	#15
Elwayno Registered User Join Date: Mar 2007 Location: Livermore, CA Posts: 34	Re: Gear ratios Earl if you change trannies and such things can be different. I was just pointing out that there is no free lunch so to speak. Not trying to be a naysayer. Gas mileage on the freeway is mostly a funtion of drag and driveline friction. In town is more to do with weight/inertia/gearing/lead foot. If going 60mph takes 50Hp with 3.73's it will take 50Hp with 3.07's. There may be very marginal gains because of the friction of the gears themselves but ignore that for a second. So if you change gears your engine slows down say 18%. Most likely your motor doesn't make as much power there so you have to push down on the gas harder. So you end up using the same gas. Now maybe you have a big block that makes a gob of torque everywhere, then maybe you don't have that problem, you lower rpm and get a gain in less loss from the part aoround the rings and piston that doesn't burn. Maybe you have a small block and you end up pushing the rpm down far enough you are off the torque peak completely and the engine is less efficient. Engines are most efficient at peak torque. As in the amount of gas used per Hp made goes down slightly so you don't want to get too far away from that. If it were jsut as ez as puttin higher gears I would get some 1.0:1's made and quadruple my gas mileage. We all know that won't work well.



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10-12-2007, 05:26 PM	#16
earl87gta Senior Member Join Date: Dec 2003 Location: Kingsport TN Posts: 4,641	Re: Gear ratios I wasnt disagreeing with you. He didnt say what trans he had now but most guys that go to a 700r4 are swaping from a t350 wich is geared almost the same as a 700r4 so it wouldnt make that big of difrenc. Now if he had a sm465 or something I would think diffrent. __________________ Earl 68 2500 4x4 GMC Burb Last edited by earl87gta; 10-12-2007 at 05:27 PM.

10-14-2007, 04:17 PM	#17
Jimmy72custom Registered User Join Date: Apr 2004 Location: Willits Ca Posts: 261	Re: Gear ratios I have a 72 Jimmy 350 / 465 / 307 / 33x12.50x15 / I run about 2200 rpm @ 65 MPH. I do notice the freeway hills. I have been thinking about a gear venders OD, to raise my 3rd gear to for the freeway hills. I have not checked my MPG, something I should do.

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